usb: phy: phy-msm-usb: Rename regulator_set_optimum_mode

The function regulator_set_optimum_mode() is changing name to
regulator_set_load(), so update the code accordingly.

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