mwl8k: ->add_interface() is not called for monitor interfaces

There is no need to check for NL80211_IFTYPE_MONITOR in
->add_interface(), as this function is never called for monitor
interfaces.  Also, there is no need to advertise this bit in our
wiphy's ->interface_modes.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>