ath10k: Use standard regulator bulk API in snoc

The regulator_get_optional() exists for cases where the driver needs do
behave differently depending on some regulator supply being present or
not, as we don't use this we can use the standard regulator_get() and
rely on its handling of unspecified regulators.

While the driver currently doesn't specify any loads the regulator
framework was updated last year to only account for load of enabled
regulators, so should the need appear it's better to apply load numbers
during initialization that dynamically.

With this the regulator wrappers have been reduced the become identical
to the standard bulk API provided by the regulator framework, so use
these instead of rolling our own.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>