Fix broken functionality by removing the bogus values in the disk structure for the stripesize, fwheads and fwsectors. On a Beaglebone Black these bogus values triggered annoying warnings (like partition not aligned to a 4M boundary due to a 4M stripe size) and worse, made the mmcsd unusable due to there being 8K sectors on a track.
There is no geometry dictated by the firmware or inherently present due to the hardware. As such, fwheads and fwsectors should never be set. These fields trigger a level of rigidity demanded by firmware and hardware that need an actual geometry. Such rigidity is inappropriate for synthesized values and are best synthesized in other places (e.g. geom_part) where they are a little less inappropriate.
On top of that, the fields in the disk structure have sectors as their unit and the mmcsd driver fills them with values that are clearly in bytes. It's the moral equivalent of being asked your height in meters and given your weight in pounds.
This change is a "back to square one" approach. One could argue that a stripe size is handy, but without due consideration of consequences, a lack of documentation and/or possibly a tunable/sysctl doing so would be ill advised. We have all the tools in place to align partitions and define appropriate file system block sizes that it's not clear why mmcsd needs to fake values.