b43legacy: Fix assigning negative value to unsigned variable

fix warning reported by smatch:
drivers/net/wireless/broadcom/b43legacy/phy.c:1181 b43legacy_phy_lo_b_measure()
warn: assigning (-772) to unsigned variable 'fval'
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/1648203433-8736-1-git-send-email-baihaowen@meizu.com

drivers/net/wireless/broadcom/b43legacy/phy.c

@@ -1123,7 +1123,7 @@ void b43legacy_phy_lo_b_measure(struct b43legacy_wldev *dev)
 	struct b43legacy_phy *phy = &dev->phy;
 	u16 regstack[12] = { 0 };
 	u16 mls;
-	u16 fval;
+	s16 fval;
 	int i;
 	int j;