hsi: fix NULL pointer in hsi_isr_tasklet

hsi: fix NULL pointer in hsi_isr_tasklet

BZ: 43597

Kernel panic at hsi_isr_tasklet when there is a tty-ffl Tx timeout.
Basically, when there is a Tx time out, tty/tty-ffl/hsi would do
an error recovery and reinitialize hsi. hsi_mid_setup calls
hsi_ctrl_set_cfg firstly to turn on irq and DMA, then calls
alloc_xfer_ctx to prepare some memory blocks. If there is a hsi interrupt,
hsi_isr_tasklet might be called before alloc_xfer_ctx.

In addition, we add BUG checking in hsi_isr_tasklet.

Change-Id: I56cc9a638d29a75b7b211416d4c413f38d93e9a4
Signed-off-by: Yanmin Zhang <yanmin.zhang@intel.com>