dpaa2-eth: free already allocated channels on probe defer

The setup_dpio() function tries to allocate a number of channels equal
to the number of CPUs online. When there are not enough DPCON objects
already probed, the function will return EPROBE_DEFER. When this
happens, the already allocated channels are not freed. This results in
the incapacity of properly probing the next time around.
Fix this by freeing the channels on the error path.

Fixes: d7f5a9d8 ("dpaa2-eth: defer probe on object allocate")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>