rt2x00: rt2800lib: fix VGC adjustment for RT3572 and RT3593

The Ralink DPO_RT5572_LinuxSTA_2.6.1.3_20121022
reference driver uses different RSSI threshold
and VGC adjustment values for the RT3572 and
RT3593 chipsets.

Update the rt2800_link_tuner function to use the
same values. Also change the comment in the function
to make it more generic.

References:

  RT35xx_ChipAGCAdjust function in chips/rt35xx.c
  RSSI_FOR_MID_LOW_SENSIBILITY constant in include/chip/rtmp_phy.h
  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>