net: macb: Fix macb mdiobus handing for DT nodes without phy-handle

commit 8ad45d02b859e31afc4cbc4ec31338ee45e94dc1 from
https://github.com/Xilinx/linux-xlnx.git

 xlnx_rebase_v5.15

When a GEM DT node contains no phy-handle property and also no child
phy nodes, the newly added shared MDIO checks are executed. Ideally,
this should have no effect but because bus_find_device finds a match
even for NULL node properties, a fake mdio producer is identified.
This leads to erroneous defer probe. Fix this by adding a NULL check
on mdio_pdev.

Fixes: a7aadd1add3f ("net: macb: In shared MDIO usecase make MDIO producer ethernet node to probe first")
Signed-off-by: Harini Katakam <harini.katakam@amd.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
State: pending
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>