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>