wl12xx: Avoid redundant TX work

TX might be handled in the threaded IRQ handler, in which case, TX work
might be scheduled just to discover it has nothing to do.

Save a few context switches by cancelling redundant TX work in case TX
is about to be handled in the threaded IRQ handler. Also, avoid
scheduling TX work from wl1271_op_tx if not needed.

Signed-off-by: Ido Yariv <ido@wizery.com>
Reviewed-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>