mt76: mt7615: Fix assigning negative values to unsigned variable

Smatch reports the following:
drivers/net/wireless/mediatek/mt76/mt7615/mac.c:1865
mt7615_mac_adjust_sensitivity() warn: assigning (-110) to unsigned
variable 'def_th'
drivers/net/wireless/mediatek/mt76/mt7615/mac.c:1865
mt7615_mac_adjust_sensitivity() warn: assigning (-98) to unsigned
variable 'def_th'

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>