amd-xgbe: Incorporate Smatch coding suggestion

The Smatch tool indicated that one of the if statements in xgbe-dev.c
could be rewritten to remove a redundant check for the 'err' variable
in an if statement.

Change the statement as suggested and add a comment to help clarify.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>