regulator: bd718x7: Use rdev_get_id() to get regulator id

Use rdev_get_id() instead of directly access rdev->desc->id.
While at it also remove unneeded init for ramp_value variable.

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