Convert dasd driver to new channel subsystem driver

This makes the dasd driver work again after the changes to
the channel subsystem driver:
- handle device detection with standard driver model functions
- reduce use of dasd_devmap (devmap will die as soon as
  dasd configuration is handled from initramfs)
- some cleanups

Authors: Arnd Bergmann <arndb@de.ibm.com>
	 Martin Schwidefsky <schwidefsky@de.ibm.com>