net: fec-mxc: prevent crash if no MAC address is set

net: fec-mxc: prevent crash if no MAC address is set

If no MAC address can be found (either in ROM, device tree or env),
the post_probe of an ethernet device will fail and therefore
the device cannot be instantiated.

The DM_ETH_PHY (eth-phy-uclass) use case must not be mixed with
using CONFIG_FEC_MXC_MDIO_BASE.

This prevents following error for MAC not fused, no eth[1]addr
env is present and providing a random MAC is disabled:

Net:
Error: ethernet@5b040000 address not set.

Error: ethernet@5b040000 address not set.
FEC: can't find phy-handle
"Error" handler, esr 0xbf000002
elr: 000000008004e4b8 lr : 000000008004e4b4 (reloc)
elr: 00000000bfe984b8 lr : 00000000bfe984b4
x0 : 0000000000000000 x1 : 0000000000000020
x2 : 00000000bbe61e50 x3 : 00000000bbe6e1a0
x4 : 0000000000000020 x5 : 0000000000000020
x6 : 000000000000000a x7 : 0000000000000000
x8 : 0000000000000000 x9 : 0000000000000008
x10: 00000000ffffffd8 x11: 0000000000000006
x12: 000000000001869f x13: 0000000000002c50
x14: 0000000000000000 x15: 00000000ffffffff
x16: 0000000000000000 x17: 0000000000000000
x18: 00000000bbe61d98 x19: 00000000bbe4fa68
x20: 00000000bbe78c10 x21: 00000000bbe6e460
x22: 00000000bbe78c10 x23: 00000000bbe91780
x24: 00000000bbe6e510 x25: 00000000000001f8
x26: 00000000ffff8000 x27: 0000000000000000
x28: 0000000000000000 x29: 00000000bbe4fa10

Code: f900003f 9100a3a1 97ffff6e 35000100 (f94017a1)
Resetting CPU ...

Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Alexander Feilke <alexander.feilke@ew.tq-group.com>