eeepc-laptop: use a mutex to serialize pci hotplug (resume vs. notify)

Commit d0265f0 "eeepc-laptop: fix hot-unplug on resume" used a workqueue
to protect pci hotplug against multiple simultaneous calls during
resume.  It seems to work, but a mutex would be more appropriate.

This is in preparation to fix the potential pci hotplug race on unload.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>