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: NTilman Schmidt <tilman@imap.cc>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/isdn/gigaset/usb-gigaset.c

@@ -39,7 +39,8 @@ MODULE_PARM_DESC(cidmode, "Call-ID mode");
 #define GIGASET_MODULENAME "usb_gigaset"
 #define GIGASET_DEVNAME    "ttyGU"
 
-#define IF_WRITEBUF 2000	/* arbitrary limit */
+/* length limit according to Siemens 3070usb-protokoll.doc ch. 2.1 */
+#define IF_WRITEBUF 264
 
 /* Values for the Gigaset M105 Data */
 #define USB_M105_VENDOR_ID	0x0681