iwlwifi: mvm: fix an overflow in iwl_mvm_get_signal_strength

The idea here is to translate a value of 0 received from
the firmware to the lowest rssi figure. As rx_status->chain_signal
is a signed byte the lowest possible value is -128 and not -256.
-256 was causing 0 to get stored in the signed byte.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>