[SCSI] fix error handling in scsi_io_completion

There was a logic fault in scsi_io_completion() where zero transfer
commands that complete successfully were sent to the block layer as
not up to date.  This patch removes the if (good_bytes > 0) gate
around the successful completion, since zero transfer commands do have
good_bytes == 0.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>