greybus: es2.c: Increase the number of bulk endpoints

ES2 support 16 endpoints. Update es2.c to allocate endpoints, urbs
and buffers for these new endpoints.

Currently, they are not yet used and es2.c is working in legacy mode
(only original endpoints are used).

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