regulator: Remove unneded check for regulator supply

The regulator_resolve_supply() function checks if a supply has been
associated with a regulator to avoid enabling it if that is not the
case.

But the supply was already looked up with regulator_resolve_supply()
and set with set_supply() before the check and both return on error.

So the fact that this statement has been reached means that neither
of them failed and a supply must be associated with the regulator.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>