Ensure to unregister all regulators before return error in probe().

The regulator register order depends on the regulator ID pass to
ab3100_regulator_register() function. Thus we need to scan ab3100_regulator_desc
and find the index of successfully registered regulators, or alternatively just
call ab3100_regulators_remove() to unregister all registered regulators.

Since current code uses a static ab3100_regulators table, explicitly set
reg->rdev = NULL after regulator_unregister() call to ensure calling
ab3100_regulators_remove() in the unwind path always work.

Also move ab3100_regulators_remove() to avoid forward declaration.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>