The debugfs support for the xpcs registers did not properly use the
specified mmd (xpcs_mmd entry) which resulted in the default mmd
value always being used.  Update the debugfs support to generate the
proper mmd register value.
Signed-off-by: NTom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>