mwl8k: move responsibility for initialising wiphy bands to GET_HW_SPEC

So that we can make 2.4 GHz and 5 GHz band registration conditional
on the capability bitmask returned by the firmware.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>