greybus: add an incoming request receive method

greybus: add an incoming request receive method

Define a new protocol method intended to handle the receipt of an
incoming operation request.  Most protocols have no expected
incoming requests and can leave this null.  If a request arrives for
a protocol with no request receive handler an error is reported and
the request fails.

Get rid of the previous fixed array of receive handlers, it's
no longer needed.

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