greybus: es2: fix endpoint requirement check

The current es2 implementation only checks for no IN or OUT bulk
endpoints, but still assumes there are precisely NUM_BULKS of each.

Specifically, urbs could be allocated and initialised to default values,
which amounts to bulk urbs being submitted to the control endpoint.

We should be able to handle any number of endpoints, but for now let's
just require NUM_BULKS of each.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>