amd-xgbe: Remove field that indicates SFP diagnostic support

The driver currently sets an indication of whether the SFP supports, and
that the driver can obtain, diagnostics data.  This isn't currently used
by the driver and the logic to set this indicator is flawed because the
field is cleared each time the SFP is checked and only set when a new SFP
is detected.  Remove this field and the logic supporting it.

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