bus: fsl-mc: return -EPROBE_DEFER when a device is not yet discovered

The fsl_mc_get_endpoint() should return a pointer to the connected
fsl_mc device, if there is one. By interrogating the MC firmware, we
know if there is an endpoint or not so when the endpoint device is
actually searched on the fsl-mc bus and not found we are hitting the
case in which the device has not been yet discovered by the bus.

Return -EPROBE_DEFER so that callers can differentiate this case.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>