mpt(4): Remove incorrect S/G segments limits.

First, two of those four checks are unreachable.
Second, I don't believe there should be ">=" instead of ">".
Third, bus_dma(9) already returns the same EFBIG if ">".

This fixes false I/O errors in worst S/G cases with maxphys >= 2MB.

Approved by: so
Security: EN-21:13.mpt
MFC after: 1 week

(cherry picked from commit 0f29396e493bd87ffa6a63fcb602b12e79d21a1e)
(cherry picked from commit f0077b4c1dcfa7eda6efadf197e8423fe002ac5d)