staging: rtl8192e: rtl_wx: fix negative noise value

wstats->qual.noise is of type uint8, so it shouldn't be assigned a
negative number. Assigning it 0x100 - 100, which is the equivalent
to -100 dBm when IW_QUAL_DBM is set.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Tested-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>