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: Rob Herring <robh@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 86c7853..21c0fab 100644
--- a/include/linux/tty.h
+++ b/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 */