Allow mac address to be set in smsc95xx

Signed-off-by: popcornmix <popcornmix@gmail.com>

SQUASH: smsc95xx: Use dev_mod_addr to set MAC addr

Since adeef3e3 ("net: constify netdev->dev_addr") it has been
illegal to write to the dev_addr MAC address field. Later commits
have added explicit checks that it hasn't been modified by nefarious
means. The dev_addr_mod helper function is the accepted way to change
the dev_addr field, so use it.

Squash with 96c1def6 ("Allow mac address to be set in smsc95xx").

Signed-off-by: Phil Elwell <phil@raspberrypi.com>