mwifiex: cleanup in mwifiex_fill_cap_info()

Pass 'struct ieee80211_ht_cap' pointer to
mwifiex_fill_cap_info() instead of
'struct mwifiex_ie_types_htcap' pointer, because the routine
internally uses the later one.
This patch also adds WARN_ON_ONCE check for NULL band.

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>