rt2800: 5592: RF early registers initialization

Based on:
NICInitRT5592RFRegisters()
RF5592Reg_2G_5G[]

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

This patch also merge common frequency adjustment (RF_R17 settings)
code. Further work is needed, to setup more RF/BBP/MAC registers after
that.

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>