Staging: ipack/devices/ipoctal: Clean up device removal.

Make use of dev_set_drvdata() and dev_get_drvdata() to store and obtain
a pointer to the ipoctal struct corresponding to a struct dev.
Previously we relied on a private list of devices.

Signed-off-by: Jens Taprogge <jens.taprogge@taprogge.org>
Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>