In mmcsd_task(), bio_resid was not being set to 0 on a successful read

or write, resulting in random short-read and short-write returns for
requests. Fixing this fixes nominal block I/O via mmcsd(4).

Obtained from:	DragonFlyBSD (fd4b97583be1a1e57234713c25f6e81bc0411cb0)
MFC after:	5 days

sys/dev/mmc/mmcsd.c

@@ -545,6 +545,8 @@ mmcsd_task(void *arg)
 			bp->bio_error = EIO;
 			bp->bio_resid = (end - block) * sz;
 			bp->bio_flags |= BIO_ERROR;
+		} else {
+			bp->bio_resid = 0;
 		}
 		biodone(bp);
 	}