staging: fsl-dpaa2/eth: Defer probing if no MC portal available

MC portals may not be available at the initial probing attempt
due to dependencies on other modules.

Check the return value of the MC portal allocation function and
defer probing in case it's not available yet. For all other error
cases the behaviour stays the same.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Suggested-by: Nipun Gupta <nipun.gupta@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>