scsi: document scsi_try_to_abort_cmd

scsi_try_to_abort_cmd() should only return SUCCESS, FAILED, or
FAST_IO_FAIL. So document that in the function description and simplify
the logging message.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>