Teach if_smsc to get MAC from bootargs.

Some Raspberry Pi pass smsc95xx.macaddr=XX:XX:XX:XX:XX:XX as bootargs.
Use this if no ethernet address is found in an EEPROM.
As last resort fall back to ether_gen_addr() instead of random MAC.

PR: 274092
Reported by: Patrick M. Hausen (via ML)
Reviewed by: imp, karels, zlei
Tested by: Patrick M. Hausen
Approved by: karels
MFC after: 1 month
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D42463