acpi_iort: fix mapping end calculation

According to the ARM Design Document "IO Remapping Table Platform"
(DEN 0049D), the "Number of IDs" field of the ID mapping format means
"The number of IDs in the range minus one".

Submitted by: Greg V <greg@unrelenting.technology>
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D25179