USB: tty: fix incorrect use of tty_insert_flip_string_fixed_flag

Fix regression introduced by commit
a108bfcb

 (USB: tty: Prune uses of
tty_request_room in the USB layer) which broke three drivers
(cypress_m8, digi_acceleport and spcp8x5) through incorrect use of
tty_insert_flip_string_fixed_flag.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@kernel.org>  [.34]
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>