ccwgroup: Unify parsing for group attribute.

Instead of having each driver for ccwgroup slave device parsing the
input itself and calling ccwgroup_create(), introduce a new function
ccwgroup_create_from_string() and handle parsing inside the ccwgroup
core.

Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>