AmpereAltraPkg/Ac01PcieLib: fix compilation error

Commit 380b4b40 ("AmpereAltraPkg: Update Ampere specific platform PCIe
core", 2023-03-29) removed the DEBUG macro call from
Ac01PcieCoreUpdateLink() that was the only reader of "Val". Since then,
Val has only been set, but never read. This triggers
"-Werror=unused-but-set-variable".

Found when trying to build "Jade.dsc".

Keep the MmioRead32() call, so that the PCIe config space sees the same
behavior as before, but explicitly throw away the result.

Cc: Chuong Tran <chuong@os.amperecomputing.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Nhi Pham <nhi@os.amperecomputing.com>
Cc: Rebecca Cran <rebecca@os.amperecomputing.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=4564


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>