This avoids lost of presence info on module reloading.
The presence info used to be only updated at the (rare) hotplug events.

Proposed by David, thanks!

CC: David Härdeman <david@hardeman.nu>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>