mwifiex: pass correct band parameter to ieee80211_channel_to_frequency()

ieee80211_channel_to_frequency() routine expects band parameter in
the form of "enum ieee80211_band band". Currently driver specific
band (BAND_A, BAND_AN etc.) is passed to the routine.

This patch makes sure that correct parameter is passed.

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>