net: sh_eth: Separate out MAC address programming

Pull out the code for writing MAC address into the NIC into a
separate function, so it can be reused by both DM and non-DM
code. This is done in preparation for DM support, which handles
MAC address programming separately.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Joe Hershberger <joe.hershberger@ni.com>