rt2x00: rt2800lib: fix default VGC values for RT3572 for the 5GHz band

The rt2x00 driver uses 0x22 as a default VGC value
in VGC adjustment for the RT3572 chipset. In the
Ralink DPO_RT5572_LinuxSTA_2.6.1.3_20121022 driver,
this value is only used for initialization. During
VGC adjustment, the reference driver uses different
values.

Update the 'rt2800_get_default_vgc' function to
synchronize the values with the reference driver.
Also add the missing AGC initialization code into
the 'rt2800_config_channel' function.

References:
  RT35xx_SetAGCInitValue in chip/rt35xx.c
  RT35xx_ChipAGCAdjust in chip/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>