net: smsc95xx: fix DM MAC address reading

eth-uclass.c expects DM-capable Ethernet adapters to implement ops->
read_rom_hwaddr(), or for some other mechanism to set pdata->enetaddr, or
for the user to set environment variable $usbethaddr. Without any of
these, it will refuse to initialize the device since no valid MAC address
is known. Implement this function for the smsc95xx driver.

With this feature implemented, there is no point smsc95xx_init_common()
re-reading the MAC address from ROM, so ifdef out this code when DM_ETH
is enabled.

This allows (at least) the built-in Ethernet on the NVIDIA Harmony board
to operate again.

Fixes: 0990fcb7 ("net: smsc95xx: Add driver-model support")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>