driver:wlan: wifi rssi adjust

Because of path loss of hardware,the returned rssi value
of wifi is not exactly. To ensure the signal strength
returned to framwork and application is exactly. the rssi
value need to be adjusted.

rssi value below -50dbm, adjust +4db.
rssi value above -50dbm, adjust +2db.

Change-Id: I7f390b1fe99bb06f481a82659dc9bdd205810bfd
Signed-off-by: Zheng Guolin <zhengguolin@huawei.com>