ath10k: fix Tx DMA alloc failure during continuous wifi down/up

ath10k: fix Tx DMA alloc failure during continuous wifi down/up

With maximum number of vap's configured in a two radio supported
systems of ~256 Mb RAM, doing a continuous wifi down/up and
intermittent traffic streaming from the connected stations results
in failure to allocate contiguous memory for tx buffers. This results
in the disappearance of all VAP's and a manual reboot is needed as
this is not a crash (or) OOM(for OOM killer to be invoked). To address
this allocate contiguous memory for tx buffers one time and re-use them
until the modules are unloaded but this results in a slight increase in
memory footprint of ath10k when the wifi is down, but the modules are
still loaded. Also as of now we use a separate bool 'tx_mem_allocated'
to keep track of the one time memory allocation, as we cannot come up
with something like 'ath10k_tx_{register,unregister}' before
'ath10k_probe_fw' is called as 'ath10k_htt_tx_alloc_cont_frag_desc'
memory allocation is dependent on the hw_param 'continuous_frag_desc'

a) memory footprint of ath10k without the change

lsmod | grep ath10k
ath10k_core           414498  1 ath10k_pci
ath10k_pci             38236  0

b) memory footprint of ath10k with the change

ath10k_core           414980  1 ath10k_pci
ath10k_pci             38236  0

Memory Failure Call trace:

hostapd: page allocation failure: order:6, mode:0xd0
 [<c021f150>] (__dma_alloc_buffer.isra.23) from
[<c021f23c>] (__alloc_remap_buffer.isra.26+0x14/0xb8)
[<c021f23c>] (__alloc_remap_buffer.isra.26) from
[<c021f664>] (__dma_alloc+0x224/0x2b8)
[<c021f664>] (__dma_alloc) from [<c021f810>]
(arm_dma_alloc+0x84/0x90)
[<c021f810>] (arm_dma_alloc) from [<bf954764>]
(ath10k_htt_tx_alloc+0xe0/0x2e4 [ath10k_core])
[<bf954764>] (ath10k_htt_tx_alloc [ath10k_core]) from
[<bf94e6ac>] (ath10k_core_start+0x538/0xcf8 [ath10k_core])
[<bf94e6ac>] (ath10k_core_start [ath10k_core]) from
[<bf947eec>] (ath10k_start+0xbc/0x56c [ath10k_core])
[<bf947eec>] (ath10k_start [ath10k_core]) from
[<bf8a7a04>] (drv_start+0x40/0x5c [mac80211])
[<bf8a7a04>] (drv_start [mac80211]) from [<bf8b7cf8>]
(ieee80211_do_open+0x170/0x82c [mac80211])
[<bf8b7cf8>] (ieee80211_do_open [mac80211]) from
[<c056afc8>] (__dev_open+0xa0/0xf4)
[21053.491752] Normal: 641*4kB (UEMR) 505*8kB (UEMR) 330*16kB (UEMR)
126*32kB (UEMR) 762*64kB (UEMR) 237*128kB (UEMR) 1*256kB (M) 0*512kB
0*1024kB 0*2048kB 0*4096kB = 95276kB

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>