net: Fix static checker warnings

Here are the smatch warning messages:

drivers/net/xilinx_axi_emac.c:324 axiemac_phy_init()
error: 'phydev' dereferencing possible ERR_PTR()

drivers/net/zynq_gem.c:340 zynq_phy_init()
error: 'priv->phydev' dereferencing possible ERR_PTR()

Fix by adding error checking before dereferencing the pointer.

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Link: https://lore.kernel.org/r/20220929045605.23964-1-venkatesh.abbarapu@amd.com


Signed-off-by: Michal Simek <michal.simek@amd.com>