[scsi] Include sense key within error number reported to user

The sense key gives a first idea of what the problem might be, and so
is potentially useful in diagnosing problems in a non-debug build.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/scsi.h

@@ -267,6 +267,9 @@ struct scsi_sns {
 	uint32_t info;
 };
 
+/** SCSI sense key mask */
+#define SCSI_SENSE_KEY_MASK 0x0f
+
 /** A SCSI response information unit */
 struct scsi_rsp {
 	/** SCSI status code */