regulator: core: Slightly improve readability of _regulator_get_enable_time

The logic is equivalent, but it looks more straightforward this way:
If rdev->desc->ops->enable_time is set, call it.
Otherwise fallback to return rdev->desc->enable_time.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>