rt2x00: Remove DRIVER_SUPPORT_MIXED_INTERFACES

A lot more is needed to support mixed interfaces then just this flag,
such things need new redesigns in rt2x00lib to actually make it happen.
So far there doesn't exist a Ralink chipset that can handle it, so
we might as well remove the flag for now and see what should be done
whenever a device appears that can handle it.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>