HID: i2c-hid: Allow ACPI systems to specify "post-power-on-delay-ms"

The property "post-power-on-delay-ms" allows a platform to specify
the delay needed after power-on, but only via device trees currently.
Use device_property_* instead of of_* reads to allow ACPI systems to
also provide the same information. This is useful for Wacom hardware
on ACPI systems.

Signed-off-by: Rajat Jain <rajatja@google.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>