[SCSI] SCSI st: fix error handling in module init, sysfs

- Notice and handle sysfs errors in module init, tape init

- Properly unwind errors in module init

- Remove bogus st_sysfs_class==NULL test, it is guaranteed !NULL at that point

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>