iwlwifi: mvm: use a cast to calculate the last seqno from the next one

If the next seqno returned by the firmware is 0, we return an error
(-16) in the iwl_mvm_get_last_nonqos_seq() function.  This is because
we return an integer and don't use any casting when calculating the
last seqno from the one we received.  Fix this by using a cast to u16
when doing the calculation, so we return 0xfff0, as we should.
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>

drivers/net/wireless/iwlwifi/mvm/d3.c

@@ -895,7 +895,7 @@ static int iwl_mvm_get_last_nonqos_seq(struct iwl_mvm *mvm,
 		/* new API returns next, not last-used seqno */
 		if (mvm->fw->ucode_capa.flags &
 				IWL_UCODE_TLV_FLAGS_D3_CONTINUITY_API)
-			err -= 0x10;
+			err = (u16) (err - 0x10);
 	}
 
 	iwl_free_resp(&cmd);