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: Rafał Miłecki <zajec5@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>