rt2x00: Fix and fine-tune rf registers for RT3070/RT3071/RT3090

Basically fix and fine-tune RT3070/RT3071/RT3090 chip RF initial value
when call rt2800_init_rfcsr

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