[SCSI] Improve SCSI_LOGGING Kconfig entry

The Kconfig entry for SCSI_LOGGING refers the reader to
drivers/scsi/scsi.c, but I didn't find any useful information
there. There is certainly logging code in that file, but the
logging types and logging levels are described in
drivers/scsi/scsi_logging.h.

Also, the procfs file referred to in the section is incorrect.
It should be /proc/sys/dev/scsi/logging_level and not
/proc/scsi/scsi.

Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>