net: micrel : ks8851-ml: add vdd-supply support

Few platforms use external regulator to keep the ethernet MAC supplied.
So, request and enable the regulator for driver functionality.

Fixes: 66fda75f (regulator: core: Replace direct ops->disable usage)
Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Suggested-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>