mfd: rk808: Ensure suspend/resume hooks always work

The RK809/RK817 suspend/resume hooks should not have to depend on
whether this driver owns the pm_power_off hook, and thus the global
rk808_i2c_client is set - indeed, the GPIO-based control is really
only relevant when PSCI firmware is in charge of power rather than
the kernel. As driver model callbacks, they have an appropriate
device argument to hand, so can just always use that.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>