s390/sclp: get rid of sclp_get_mtid() and sclp_get_mtid_max()

As all relevant sclp data is now directly accessible, let's move the
logic of these two functions to the single caller.

Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>