amd-xgbe: Check xgbe_init() return code

The xgbe_init() routine returns a return code indicating success or
failure, but the return code is not checked. Add code to xgbe_init()
to issue a message when failures are seen and add code to check the
xgbe_init() return code.

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