s390/scm_block: do not hide eadm subchannel dependency

Stop hiding scm_block's dependency to the eadm subchannel driver
(by using functions provided by the eadm subchannel instead of
wrappers provided by the scm bus).

This will help userspace recognizing module dependencies (e.g. for
building a ramdisk). As a side effect we can get rid of some code
reimplementing refcounting between those modules.

Reported-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Reviewed-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>