rt2x00: rt2800lib: fix VGC programming for RT3572 and RT3593

According to the DPO_RT5572_LinuxSTA_2.6.1.3_20121022
reference driver, programming of the 'BBP 66' register
on the RT3572 and RT3593 chipsets must be done via the
'rt2800_bbp_write_with_rx_chain' function. This ensures
that value is correclty set for all RX chains.

References:
  RT35xx_ChipAGCAdjust and RT35xx_SetAGCInitValue functions
  in chips/rt35xx.c

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>