regulator: lp8755: Fix lp8755_regulator_init unwind code

It's safe to pass NULL argument to regulator_unregister(), so we can remove
the NULL checking before calling regulator_unregister().
However pass a ERR_PTR to regulator_unregister() is wrong, so we need to
explicitly set "pchip->rdev[buck_num] = NULL" before goto err_buck.

This patch also includes below cleanups:
Show correct regulator id in dev_err.
Remove __devexit_p.

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