net: ks8851: Don't use regulator_get_optional()

We shouldn't be using regulator_get_optional() here. These
regulators are always present as part of the physical design and
there isn't any way to use an internal regulator or change the
source of the reference voltage via software. Given that the only
users of this driver in the kernel are DT based, this change
should be transparent to them even if they don't specify any
supplies because the regulator framework will insert dummy
supplies as needed.

Cc: Nishanth Menon <nm@ti.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: Mark Brown <broonie@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>