[SCSI] sr: fix test unit ready responses

Commit 210ba1d1724f5c4ed87a2ab1a21ca861a915f734 updated sr.c to use
the scsi_test_unit_ready() function.  Unfortunately, this has the
wrong characteristic of eating NOT_READY returns which sr.c relies on
for tray status.

Fix by rolling an internal sr_test_unit_ready() that doesn't do this.

Tested-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>