regulator: isl6271a: Remove *rdev[3] from struct isl_pmic

This driver is using devm_regulator_register, so it's not necessary to
store *rdev[3] in struct isl_pmic. Use a local variable instead.

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