rt2800: 5592: common BBP initialization

Add BBP registers initialization common with other chipsets, but for now
performed only for 5592.

Based on:
NICInitBBP()

from:
DPO_RT5572_LinuxSTA_2.6.1.3_20121022/common/rtmp_init.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>