Revert "[ahci] Port memory configuration math cleanup."

try revert and check if this is making zircon builders timeout

This reverts commit 157b4786.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [ahci] Port memory configuration math cleanup.
> 
> No functionality changes.
> 
> Replace pointer math with easier to follow structure describing the
> port command memory layout.
> 
> Test: blktest
> Change-Id: Ia85bce4224e20497da51e4bee6fb962cfb54a986

TBR=smklein@google.com,qsr@google.com,sron@google.com,auradkar@google.com

Change-Id: I555908e7ba0e8ee1b84935ded01284034f4ecea7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true