USB: cdc-wdm: better allocate a buffer that is at least as big as we tell the USB core

As it turns out, there was a mismatch between the allocated inbuf size
(desc->bMaxPacketSize0, typically something like 64) and the length we
specified in the URB (desc->wMaxCommand, typically something like 2048)

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Cc: Oliver Neukum <oliver@neukum.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>