wiznet: Fix Kconfig dependencies.

Both drivers need to depend upon HAS_IOMEM, otherwise we
get a build failure on platforms like S390.

All the driver specific config options need to depend upon
the drivers themselves.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>