staging: rtl8192u: r8192U_wx: fix negative noise and level values

range->max_qual.noise and level are of type uint8, so they shouldn't be
assigned a negative number. Assigning them 0x100 - 98, and 0x100 - 78 which
are the equivalent to -98 dBm and -78 dBm, respectively, 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>