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

This reverts commit 4d833f15.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> 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

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

Change-Id: I2d43c5f23ff938282ba018a449ad52274f6ef8d8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true