staging: rtl8712: fix negative level value

range->avg_qual.level is of type uint8, so it shouldn't be assigned a
negative number. Assigning it 0x100 - 78, which is the equivalent
to (20 + -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>