firmware: arm_scmi: Fix signed error return values handling

Handle signed error return values returned by simple_write_to_buffer().
In case of an error, return the error code.

Fixes: 3c3d818 ("firmware: arm_scmi: Add core raw transmission support")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Sukrut Bellary <sukrut.bellary@linux.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20230718085529.258899-1-sukrut.bellary@linux.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

Author: sukrutb

drivers/firmware/arm_scmi/raw_mode.c

@@ -818,10 +818,13 @@ static ssize_t scmi_dbg_raw_mode_common_write(struct file *filp,
 	 * before sending it with a single RAW xfer.
 	 */
 	if (rd->tx_size < rd->tx_req_size) {
-		size_t cnt;
+		ssize_t cnt;
 
 		cnt = simple_write_to_buffer(rd->tx.buf, rd->tx.len, ppos,
 					     buf, count);
+		if (cnt < 0)
+			return cnt;
+
 		rd->tx_size += cnt;
 		if (cnt < count)
 			return cnt;