wlcore: fix unsafe dereference of the wlvif

wlvif could be passed as NULL from the wlcore_tx_work_locked()
to the wl1271_prepare_tx_frame() and to wl1271_skb_queue_head()
functions. This may lead to a Kernel panic, fix this by
validating that wlvif != NULL.

Signed-off-by: Victor Goldenshtein <victorg@ti.com>