[SCSI] quieten messages on scsi_execute commands

scsi_io_completion() can be a bit noisy about certain conditions.
Previously this wasn't a problem for internally generated commands,
since they never hit it.  However, since we do all SCSI commands via
bios, now they do.  user CD testers like magicdev are now getting not
ready messages every time they touch the CD to see if there's anything
in it.

Fix this by making all scsi_execute commands REQ_QUIET and making
scsi_finish_io() not say anything for REQ_QUIET.

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