net: smc911x: Keep MAC programmed

Make sure to keep the MAC address programmed in the SMC911x ADDRH
and ADDRL registers. Linux can read those registers to determine
the MAC address on EEPROM-less configurations.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Tom Rini <trini@ti.com>