USB: cdc-wdm: Avoid hanging on interface with no USB_CDC_DMM_TYPE

USB: cdc-wdm: Avoid hanging on interface with no USB_CDC_DMM_TYPE

The probe does not strictly require the USB_CDC_DMM_TYPE
descriptor, which is a good thing as it makes the driver
usable on non-conforming interfaces.  A user could e.g.
bind to it to a CDC ECM interface by using the new_id and
bind sysfs files.  But this would fail with a 0 buffer length
due to the missing descriptor.

Fix by defining a reasonable fallback size: The minimum
device receive buffer size required by the CDC WMC standard,
revision 1.1

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>