Check for and handle failures of bus_dmamap_load(). The driver currently
requires that each 512 byte IO be in a single contiguous buffer, but if a
buffer crosses a page boundary and the physical pages aren't contiguous
you can get an EFBIG failure (too many segments).

The driver really should handle multiple segment IO, but before adding that
I wanted to make sure that it's handling failure properly while the failure
is easily recreatable.