isdn/gigaset: adjust usb_gigaset tty write buffer limit

The usb_gigaset driver's write buffer limit was different from those
of the others for no good reason. Set it to the same value, derived
from the Siemens documentation.

Impact: cosmetic
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>