ufs: Rename of regulator_set_optimum_mode

The function regulator_set_optimum_mode() is changing name to
regulator_set_load(), so update the code accordingly. Also cleaned up
ufshcd_config_vreg_load() while touching the code.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Mark Brown <broonie@kernel.org>