rt2x00: Fix partial antenna configuration

The if-statement to determine the new TX/RX antenna
configuration was incomplete. It lacks the general
else-clause when the antenna wasn't changed.

This is a correct event, since it can occur when only
one of the antenna's has been changed or when the new
configuration is being forced (like when the interface
has just been added).

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