platform/x86: asus-wmi: Refactor egpu_enable attribute

platform/x86: asus-wmi: Refactor egpu_enable attribute

The settings for these attributes can be read from the device, this
is now done instead of reading a stored value from module. The stored
value is also removed.

This means the simpler asus_wmi_dev_is_present() can be used in
*_check_present() - it is not an error for these methods to be
missing.

The _write() functions have their bodies shifted in to *_store()
which simplifies things further.

Signed-off-by: Luke D. Jones <luke@ljones.dev>
Link: https://lore.kernel.org/r/20220812222509.292692-6-luke@ljones.dev
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>