wlcore/wl12xx/wl18xx: simplify fw_status handling

Instead of splitting the fw_status into 2 and using some
complex calculations, read the fw status and let each low-level
driver (wl12xx/wl18xx) convert it into a common struct.

This is required for the upcoming fw api changes, which
break the current logic anyway.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>