rt2x00: Fix MCS register intialization

According to the original Ralink driver,
LG_FBK_CFG0_OFDMMCS2FBK should be initialized to 9 rather then 3.

Based on a patch for rt2800pci from Benoit

Signed-off-by: Benoit PAPILLAULT <benoit.papillault@free.fr>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>