r8152: add missing endpoint sanity check

Add missing endpoint sanity check to probe in order to prevent a
NULL-pointer dereference (or slab out-of-bounds access) when retrieving
the interrupt-endpoint bInterval on ndo_open() in case a device lacks
the expected endpoints.

Fixes: 40a82917 ("net/usb/r8152: enable interrupt transfer")
Cc: hayeswang <hayeswang@realtek.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>