greybus: uart: properly calculate max buffer size

We forgot to count the size of the uart send data message header when
calculating the maximum size of the buffer that the uart driver could
send in one chunk.

This fixes the math and makes the variable a size_t to match the return
value of the call to gb_operation_get_payload_size_max();
Reported-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Tested-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/uart.c

@@ -43,7 +43,7 @@ struct gb_tty_line_coding {
 struct gb_tty {
 	struct tty_port port;
 	void *buffer;
-	u32 buffer_payload_max;
+	size_t buffer_payload_max;
 	struct gb_connection *connection;
 	u16 cport_id;
 	unsigned int minor;
@@ -608,11 +608,8 @@ static int gb_uart_connection_init(struct gb_connection *connection)
 	}
 
 	gb_tty->buffer_payload_max =
-		gb_operation_get_payload_size_max(connection);
-	if (!gb_tty->buffer_payload_max) {
-		retval = -EINVAL;
-		goto error_payload;
-	}
+		gb_operation_get_payload_size_max(connection) -
+			sizeof(struct gb_uart_send_data_request);
 
 	gb_tty->buffer = kzalloc(gb_tty->buffer_payload_max, GFP_KERNEL);
 	if (!gb_tty->buffer) {