rt2x00: Style fix for interval defines

Extra parenthesis are not needed in these 2 cases,
all other defines in rt2x00 are done without parenthesis
so just fixup these 2 cases.

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