mwifiex: use IEEE80211_HT_CAP_ macros for 11n cap_info

The hw_dot_11n_dev_cap reported by firmware hw_spec
has different format than the 11n capabilities.
Hence a lot of SET_ and RESET_ bit operation macros
were used to convert the dev_cap format to 11n
capability format. However the locally defined 11n
ht_cap macros are not necessary as we can use
IEEE80211_HT_CAP_ macros directly.

The 32-bit dev_cap bitmap is added as comment to
explain the mapping between firmware and 11n spec.

Some unused macros and unnecessary adapter variables
are also removed.

Signed-off-by: Marc Yang <yangyang@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>