eeepc-wmi: use the presence bit correctly

I checked some more DSDT, and it seems that I wasn't
totally right about the meaning of DSTS return value.
Bit 0 is clearly the status of the device, and I discovered
that bit 16 is set when the device is present.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Matthew Garrett <mjg@redhat.com>