tty: constify tty_ldisc_receive_buf buffer pointer

This is needed to work with the client operations which uses const ptrs.

Really, the flags pointer could be const, too, but this would be a tree
wide fix.
Signed-off-by: NRob Herring <robh@kernel.org>
Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-By: NSebastian Reichel <sre@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_buffer.c

@@ -422,7 +422,7 @@ EXPORT_SYMBOL_GPL(tty_prepare_flip_string);
  *
  *	Returns the number of bytes not processed
  */
-int tty_ldisc_receive_buf(struct tty_ldisc *ld, unsigned char *p,
+int tty_ldisc_receive_buf(struct tty_ldisc *ld, const unsigned char *p,
 			  char *f, int count)
 {
 	if (ld->ops->receive_buf2)

include/linux/tty.h

@@ -657,7 +657,7 @@ extern int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty);
 extern void tty_ldisc_release(struct tty_struct *tty);
 extern void tty_ldisc_init(struct tty_struct *tty);
 extern void tty_ldisc_deinit(struct tty_struct *tty);
-extern int tty_ldisc_receive_buf(struct tty_ldisc *ld, unsigned char *p,
+extern int tty_ldisc_receive_buf(struct tty_ldisc *ld, const unsigned char *p,
 				 char *f, int count);
 
 /* n_tty.c */