iwlwifi: make tx_cmd_pool kmem cache global

iwlwifi: make tx_cmd_pool kmem cache global

Otherwise we are not able to run more than one device per driver:

[   24.743045] kmem_cache_create: duplicate cache iwl_dev_cmd
[   24.743051] Pid: 3165, comm: NetworkManager Not tainted 3.3.0-rc2-wl+ #5
[   24.743054] Call Trace:
[   24.743066]  [<ffffffff811717d5>] kmem_cache_create+0x655/0x700
[   24.743101]  [<ffffffffa03b9f8b>] iwl_alive_notify+0x1cb/0x1f0 [iwlwifi]
[   24.743111]  [<ffffffffa03ba442>] iwl_load_ucode_wait_alive+0x1b2/0x220 [iwlwifi]
[   24.743142]  [<ffffffffa03ba893>] iwl_run_init_ucode+0x73/0x100 [iwlwifi]
[   24.743152]  [<ffffffffa03b8fa1>] __iwl_up+0x81/0x220 [iwlwifi]
[   24.743161]  [<ffffffffa03b91c0>] iwlagn_mac_start+0x80/0x190 [iwlwifi]
[   24.743188]  [<ffffffffa03307b3>] ieee80211_do_open+0x293/0x770 [mac80211]

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>