power: ds2782_battery: Simplify the PM hooks

The SIMPLE_DEV_PM_OPS() macro already takes care of the CONFIG_PM_SLEEP=n case,
so we can simplify the code a little bit.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>