mwifiex: HT capability information handling

1) Initialise HT capabilities in cfg80211 properly.
2) Cfg80211 stack may modify "sband->ht_cap" to disable
40Mhz operation in 2.4GHz band (after recent patch
"cfg80211: module_param to disable HT40 in 2.4GHz band")
Therefore read "sband->ht_cap" instead of an adapter variable
"hw_dot_11n_dev_cap" to get HT capabilities.

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>