staging: rtl8192u: r8192U_core: fix negative noise value

ieee80211_rx_stats.noise is of type uint8, so it shouldn't be assigned a
negative number. Assigning it 0x100 - 98, which is the equivalent
to -98 dBm when IW_QUAL_DBM is set.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>