b43: Fix assigning negative value to unsigned variable

fix warning reported by smatch:
drivers/net/wireless/broadcom/b43/phy_n.c:585 b43_nphy_adjust_lna_gain_table()
warn: assigning (-2) to unsigned variable '*(lna_gain[0])'
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/1648203315-28093-1-git-send-email-baihaowen@meizu.com

drivers/net/wireless/broadcom/b43/phy_n.c

@@ -582,7 +582,7 @@ static void b43_nphy_adjust_lna_gain_table(struct b43_wldev *dev)
 	u16 data[4];
 	s16 gain[2];
 	u16 minmax[2];
-	static const u16 lna_gain[4] = { -2, 10, 19, 25 };
+	static const s16 lna_gain[4] = { -2, 10, 19, 25 };
 
 	if (nphy->hang_avoid)
 		b43_nphy_stay_in_carrier_search(dev, 1);