net: sh_eth: Fix DT base address fetching

Drop the whole map/unmap_physmem stuff and just use the address
already obtained from DT in ofdata_to_platdata(), instead of
repeating that, wrongly, in probe.

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