platform/x86: system76_acpi: Use dev_get_drvdata

Eliminate direct accesses to the driver_data field.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1653989063-20180-1-git-send-email-baihaowen@meizu.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>