wl12xx: fix unsafe dereference of the wlvif

wlvif could be passed as NULL from the wl1271_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 before using it.

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

Signed-off-by: Michael Bestas <mikeioannina@gmail.com>

Change-Id: I92f5cfec270919efdd8780d9411fe837eafe4df4