regulator: core: Remove redundant checks

In function _regulator_do_set_voltage(), old_selector gets intialised only
if (_regulator_is_enabled(rdev) && rdev->desc->ops->set_voltage_time_sel &&
rdev->desc->ops->get_voltage_sel)) is true.

Before calling set_voltage_time_sel() we checks if (old_selector >= 0) and it
will true if it got intialised properly. so we don't need to check again
_regulator_is_enabled(rdev) && rdev->desc->ops->set_voltage_time_sel before
calling set_voltage_time_sel().

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>