greybus: activate incoming request handling

Un-comment gb_operation_request_handle(), which was recently
disabled to avoid distraction.

In gb_connection_recv_request(), activate handling incoming
requests by defining gb_operation_request_handle() as an
incoming operation's callback function.

Incoming operation requests have

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>