rt2800: 5592: MAC registers initalization

Based on:
NICInitRT5592MacRegisters()

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

On vendor driver we do not initialize TX_SW_CFG{1,2}. However the same
difference is between rt2x00 and vendor driver for 5390 chip.

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>