wl18xx: implement immediate Tx completion

Implement immediate Tx completion for the 18xx family. Move 18xx
specific Tx code to new tx.c/h files and create helper header files
for definitions.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>
7 files changed