mrsas(4): Report more correct maximum I/O size.

Subtract one SGE for the case of misaligned address. Also take into
account maximum number of sectors reported by firmware, that gives
nicer 256KB limit instead of 276KB calculated from the SGE limit.

While there, remove number of I/O size checks, duplicating what is
already checked by CAM and busdma(9).

MFC after: 1 month
Sponsored by: iXsystems, Inc.