rt2800: 5592: add chip specific vgc calculations

Based on:
RT5592_ChipAGCAdjust()

from:
DPO_RT5572_LinuxSTA_2.6.1.3_20121022/chips/rt5592.c

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>