mwifiex: correct bss_mode check while appending vht operation IE

priv->bss_mode uses NL80211_IFTYPE_* definitions.
HostCmd_BSS_MODE_IBSS is used in ad-hoc start/join command between
driver and firmware.

Coincidentally both HostCmd_BSS_MODE_IBSS and NL80211_IFTYPE_STATION
are defined as 2. That explains why nobody complained.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>