Modify the spi flash driver to allow smaller read IO sizes, but enforce
the larger, aligned write+erase sizes the driver currently implements.

This preserves write behaviour but makes the flash driver usable for things
like a read-only FFS or a geom_uzip/geom_compress .

Note that since GEOM will now return the sector size as being smaller,
writes of sector size/alignment will now fail with an EIO. Code which
writes to the flash device will have to be (for now) manually taught
about the flash write blocksize.