[PATCH] Add char_devs to st

[PATCH] Add char_devs to st
This patch adds support for cdevs to the st driver. The changes are based on
Doug Gilbert's corresponding changes to the sg driver. Using cdevs brings the
following advantanges:
- support for many drives, currently the maximum is set to 128; the minor
  number assignment is explained in the patch to Documentation/scsi/README.st
- the tape devices appear in /sys/cdev/major/st*, as an example here are
  the entries for the first drive (4 modes, rewind and non-rewind
  devices):
  /sys/cdev/major/st0m0   /sys/cdev/major/st0m1   /sys/cdev/major/st0m2
  /sys/cdev/major/st0m3
  /sys/cdev/major/st0m0n  /sys/cdev/major/st0m1n  /sys/cdev/major/st0m2n
  /sys/cdev/major/st0m3n