rt2x00: rt2800lib: fix default VGC values for RT3593

Update the rt2800_get_default_vgc function to use the same VGC
values that the DPO_RT5572_LinuxSTA_2.6.1.3_20121022 reference
driver uses.

References:
  RT35xx_ChipAGCAdjust in chips/rt35xx.c
  RT3593_R66_MID_LOW_SENS_GET macro in include/chip/rt3593.h
  RT3593_R66_NON_MID_LOW_SEMS_GET macro in include/chips/rt3593.h

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>