greybus: es2: update the bulk_ep_set value accepted by map_to_cpor_ep()

The endpoint set 0 is currently considered as invalid.
But 0 mean muxed cports on ep1 and ep2,
then it must not return -EINVAL.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>