greybus: es2: use a single bulk ep pair for all greybus data

This matches a corresponding firmware change to declare a single
BULK EP pair for all greybus traffic and 1 BULK IN ep for ARPC.

Thus, avoiding URB submittions for all the other unused BULK IN
endpoints on the HOST side that took considerable amount
of bus time.

Testing Done:
Tested with modified AP<->APB1 loopback test and also with GPBridge
modules to ensure there are no regressions

Signed-off-by: Sandeep Patil <sspatil@google.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>