regulator: core: Allow regulator_set_voltage for fixed regulators

regulator: core: Allow regulator_set_voltage for fixed regulators

Make it okay to call regulator_set_voltage on regulators with fixed
voltage if the requested range overlaps the current/configured voltage.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
(cherry picked from commit c00dc359e5e0b10de993651d8e73e60c41bf29cd)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
(cherry picked from commit d5c3595331d512c6af6b636cef687421c2ba11e3)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>