p54: better rssi->dbm conversion for longbow phys

This patch should fix the dodgy signal and noise value
reports for most longbow (p54spi) users:

e.g.: (an older tcpdump extract)
> 2.0 Mb/s 2437 MHz (0x00a0) -49dB signal -41dB noise
> 1.0 Mb/s 2437 MHz (0x00a0) 4dB signal -41dB noise

The formula is taken from stlc45xx:
http://gitorious.org/stlc45xx/mainline/blobs/master/stlc45xx.c#line1199

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>