USB: cdc-wdm: kill the now unnecessary bMaxPacketSize0 field and udev variable

We don't need bMaxPacketSize0, and keeping all these different size fields
around will only cause us to use the wrong one.

Seems the udev variable was only used for getting bMaxPacketSize0.  We
could have used it for the usb_fill_*_urb() calls, but as it wasn't
before - why start now?  Instead make the interface_to_usbdev()
calls consistent.

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