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