b43: N-PHY: use more bits for offset in RSSI calibration

When calculating "offset" for final RSSI calibration we're using numbers
bigger than s8 can hold. We have for example:
offset[j] = 232 - poll_results[j];
formula. If poll_results[j] is small enough (it usually is) we treat
number's bit as a sign bit. For example 232 - 1 becomes:
0xE8 - 0x1 = 0xE7, which is not 231 but -25.

This code was introduced in e0c9a021
and caused stability regression on some cards, for ex. BCM4322.
Signed-off-by: NRafał Miłecki <zajec5@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/b43/phy_n.c

@@ -1564,7 +1564,7 @@ static void b43_nphy_rev3_rssi_cal(struct b43_wldev *dev)
 	u16 clip_off[2] = { 0xFFFF, 0xFFFF };
 
 	u8 vcm_final = 0;
-	s8 offset[4];
+	s32 offset[4];
 	s32 results[8][4] = { };
 	s32 results_min[4] = { };
 	s32 poll_results[4] = { };
@@ -1732,7 +1732,7 @@ static void b43_nphy_rev2_rssi_cal(struct b43_wldev *dev, u8 type)
 	u8 regs_save_radio[2];
 	u16 regs_save_phy[2];
 
-	s8 offset[4];
+	s32 offset[4];
 	u8 core;
 	u8 rail;