wl1251: split RX and TX data path initialisation

Split up data path initialisation into RX and TX data path initialisation
functions. This change is required for channel switching in monitor mode.

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>