mwl8k: use firmware capability field to decide which bands to register

Make the decision about whether to register the 2.4 and 5 GHz bands
with mac80211 by looking at the capability field in GET_HW_SPEC (STA
firmware only for now).  This enables 5 GHz STA operation.

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