net: fix IBM EMAC driver after rename.

In commit 9aa32835 (ehea/ibm*: Move the
IBM drivers) the IBM_NEW_EMAC* were renames to IBM_EMAC*

The conversion was incomplete so that even if the driver was added to
the .config it wasn't built, but there were no errors).  In this commit
we also update the various defconfigs that use EMAC to use the new
Kconfig symbol, and explicitly add the NET_VENDOR_IBM guard.

We do not explicitly select the Kconfig dependencies, as this would force
EMAC on.  Doing it in the defconfig allows more flexibility.

Tested on a canyondlands board.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>