Staging: ipack: remove field driver from struct ipack_device.

After a successful match is found the driver field in struct device is
set by the core device code. We can use this field.

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