mwifiex: get_channel from firmware

At present driver gets chan_type by referring to
IEEE80211_HT_PARAM_CHA_SEC_OFFSET, in ASSOC response. Sometimes
AP shows IEEE80211_HT_PARAM_CHA_SEC_OFFSET as above/below in
assoc response, even if the association is done on HT20 channel
only. So, it will be accurate to get econdary channel offset from
firmware.

Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>