mt76: mt7615: Fix assigning negative values to unsigned variable

mt76: mt7615: Fix assigning negative values to unsigned variable

BugLink: https://bugs.launchpad.net/bugs/1969107
[ Upstream commit 9273ffcc9a11942bd586bb42584337ef3962b692 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 556ec5030e5743354c2083f621f5ff85b84dae53)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>