dwmmc: Calculate the maximum transaction length correctly.

We should reserve two descriptors (not MMC_SECTORS) for potentially
unaligned (so bounced) buffer fragments, one for the starting fragment
and one for the ending fragment.

Submitted by: kjopek@gmail.com
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30387

(cherry picked from commit dfb7360222856e7e4f5e0e5564281a25af63319c)