net: smc911x: Properly handle EEPROM MAC address

net: smc911x: Properly handle EEPROM MAC address

When compiled as a DM_ETH driver, the scm911x driver was reading the MAC
address from the optional EEPROM storage, but failed to copy this to the
platdata struct. Since it was also missing a definition of the
read_rom_hwaddr() function, the generic Ethernet code was dismissing
this MAC address, falling back to a random address or denying to start
at all.

Add an implementation of .read_rom_hwaddr, and refactor the function
reading the ROM address to be called by all interested parties.

This fixes MAC address issues when using the driver in DM_ETH "mode".

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-By: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>