wl12xx: moved firmware version reading routine to chip-specific functions

With WL1271, the firmware version can only be read right after booting the
chip.  To keep WL1251 aligned with this procedure, the code that reads the
firmware version initially has been moved to a common place where it can be
read from both chipsets.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>