regulator: core: Add checking set_mode callback in regulator_set_optimum_mode

regulator_set_optimum_mode needs set_mode to properly work.
Add checking for set_mode callback in case it may be not implemented.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>