rt2800: radio 3xxx: reprogram only lower bits of RF_R3

Synchronize code with Ralink driver:
2011_0719_RT3070_RT3370_RT5370_RT5372_Linux_STA_V2.5.0.3_DPO
(functions: RT33xx_ChipSwitchChannel() and RT30xx_ChipSwitchChannel())

Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>