amd-xgbe: Fix up some coccinelle identified warnings

Fix up some warnings that were identified by coccinelle:

Clean up an if/else block that can look confusing since the same statement
is executed in an "else if" check and the final "else" statement.

Change a variable from unsigned int to int since it is used in an if
statement checking the value to be less than 0.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>