mxser: make xmit ring buffer variables unsigned

Make the xmit ring buffer pointers and counter (xmit_head, xmit_tail,
xmit_cnt) unsigned. Now, the assumptions for the compiler are clear --
they cannot be negative.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-20-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/mxser.c

@@ -351,9 +351,9 @@ struct mxser_port {
 	u8 read_status_mask;
 	u8 ignore_status_mask;
 	u8 xmit_fifo_size;
-	int xmit_head;
-	int xmit_tail;
-	int xmit_cnt;
+	unsigned int xmit_head;
+	unsigned int xmit_tail;
+	unsigned int xmit_cnt;
 	int closing;
 
 	struct ktermios normal_termios;