mmc: core: Remove fixed voltage regulator logic

There is no need for regulator consumers to include special logic for
fixed voltage regulators as they support regulator_set_voltage() just
like their non-fixed regulator counterparts.

Signed-off-by: Tim Kryger <tim.kryger@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>