regulators: max8973: fix multiple instance support

Currently the max8973 regulator driver uses a single static struct of
regulator operations for all chip instances, but can overwrite some of its
members depending on configuration. This will affect all other MAX8973
instances on the system. This patch fixes this bug by allocating a separate
copy of the struct for each chip instance.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>