mwifiex: use IEEE80211_HT_PARAM_CHA_SEC_* macros

Replace driver specific macros with the corresponding
IEEE80211_HT_PARAM_CHA_SEC_* macros defined in ieee80211.h.
Also, rename 'adapter->chan_offset' to 'adapter->sec_chan_offset'
for consistency.

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>