gigaset: prune use of tty_buffer_request_room

Calling tty_buffer_request_room() before tty_insert_flip_string()
is unnecessary, costs CPU and for big buffers can mess up the
multi-page allocation avoidance.
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Acked-by: Karsten Keil <keil@b1-systems.de>
CC: Alan Cox <alan@lxorguk.ukuu.org.uk>, stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/gigaset/interface.c

@@ -628,7 +628,6 @@ void gigaset_if_receive(struct cardstate *cs,
 	if (tty == NULL)
 		gig_dbg(DEBUG_IF, "receive on closed device");
 	else {
-		tty_buffer_request_room(tty, len);
 		tty_insert_flip_string(tty, buffer, len);
 		tty_flip_buffer_push(tty);
 	}