rtl8180: Fix to add STA mode

To be compatible with mac80211 following "mac80211: only create
default STA interface if supported", rtl8180 needs to set
NL80211_IFTYPE_STATION in interface_modes.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Reported-by: Fabio Rossi <rossi.f@inwind.it>
Tested-by: Piter PUNK <piterpunk@unitednerds.org>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>