rt2x00: Code clean up in rt2800lib.

Make the code in rt2800_get_default_vgc more understandable and
readable, especially for the 5GHz band values.

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