carl9170: fix noise dBm conversion

Ever since carl9170 gained support to read the noisefloor,
the reported noisefloor level was pretty poor.

Initially I assumed that something was wrong in the PHY
setup and it would be impossible to fix without any
guidances. But this was not the case. In fact the nf
readings were correct and the thing that was broken
was the "simple" sign extension code!

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