amd-xgbe: Fix the xpcs mmd debugfs support

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: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>