tty: localise ptychar and make it const

ptychar was not const, so mark it as such. And move this variable to the
only place where it's used.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-36-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1196,8 +1196,6 @@ int tty_send_xchar(struct tty_struct *tty, char ch)
 	return 0;
 }
 
-static char ptychar[] = "pqrstuvwxyzabcde";
-
 /**
  *	pty_line_name	-	generate name for a pty
  *	@driver: the tty driver in use
@@ -1211,6 +1209,7 @@ static char ptychar[] = "pqrstuvwxyzabcde";
  */
 static void pty_line_name(struct tty_driver *driver, int index, char *p)
 {
+	static const char ptychar[] = "pqrstuvwxyzabcde";
 	int i = index + driver->name_base;
 	/* ->name is initialized to "ttyp", but "tty" is expected */
 	sprintf(p, "%s%c%x",