uefi: disable plugin if efivar is not supported
uefi: disable plugin if efivar is not supported

Don't even try to coldplug the device.

Fixes: #2237

hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd