[S390] cio: Make ccw_dev_id_is_equal() more robust.

Using memcmp to compare ccw_dev_id implies that the whole structure (incl.
padding) has always been completely initialized to sane values. Comparing
the structures field by field doesn't make such assumptions.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>