mwifiex: check firmware capabilities while initialising 5GHz band parameters

There are some SD8787 cards which don't support 5GHz band.
Therefore initialise 5GHz band parameters only if hardware
supports the band.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>