MdePkg Protocol/AtaPassThru.h: Update PortMultiplierPort related comments

MdePkg Protocol/AtaPassThru.h: Update PortMultiplierPort related comments

The UEFI2.5 Errata (Mantis ticket 1472 -
https://mantis.uefi.org/mantis/view.php?id=1472) updated
'PortMultiplierPort' parameter description for the following services in
EFI_ATA_PASS_THRU_PROTOCOL:

1. PassThru
2. BuildDevicePath
3. ResetDevice

Now, if there is no port multiplier on a ATA controller port, the
PortMultiplierPort parameter should be set to 0xFFFF instead of 0.

Please note that this is an incompatible semantic change. The consumer of
EFI_ATA_PASS_THRU_PROTOCOL needs to re-examine its usage to follow UEFI
2.5 Errata mantis 1472.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>