rt2x00: Fix typo in rf programming of rt2800lib.

Fix a type in rt2800_config_channel_rt3x. The second write to RF
register 2 should be to RF register 3.  This is confirmed by the
legacy Ralink code.

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>