regulator: Rename regulator_set_optimum_mode

Rename the regulator_set_optimum_mode() function regulator_set_load() to
better represent what's going on.

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