powerpc64: fix the calculation of Maxmem

[luporl]

Summary

The calculation of Maxmem was skipping the last phys_avail segment,
because of a wrong stop condition.

This was detected when using QEMU/PowerNV with Radix MMU and
low memory (2G). In this case opal_pci would allocate a DMA window
that was too small to cover all physical memory, resulting in reading all
zeroes from disk when using memory that was not inside the allocated
window.

Test Plan

Tested on QEMU/PowerNV, S812L (POWER8) and Talos II (POWER9).