rt2x00: Add unlikely macro to special case tx status handling

This special case shouldn't happen very often. Only if a frame that
is not intended to be aggregated ends up in an AMPDU _and_ was intended
to be sent at a different MCS rate as the aggregate. Hence, using
unlikely is justified.

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