Fix scsi_cdb_read6()

The maximum number of blocks that can be transferred at once via a READ6
command is 256 instead of 265.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

lib/scsi-lowlevel.c

@@ -1400,7 +1400,7 @@ scsi_cdb_read6(uint32_t lba, uint32_t xferlen, int blocksize)
 	int num_blocks;
 
 	num_blocks = xferlen/blocksize;
-	if (num_blocks > 265) {
+	if (num_blocks > 256) {
 		return NULL;
 	}