wl12xx: don't modify the global supported band structures

When 11a is not supported, we were modifying the global structure that
contains the bands supported by the driver.  This causes problems when
having more one wl12xx device in the same system because they all use
the same global.  This also causes problems when the wl12xx_sdio
module is removed and the wl12xx module remains.

Fix this problem by copying the band structure into the wl12xx
instance.

Reported-by: Arik Nemtsov <arik@wizery.com>
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Luciano Coelho <coelho@ti.com>