The Flow Control selftest is also available with ASYM Pause. Lets add
this check to the test and fix eventual false positive failures.

Fixes: 091810db

 ("net: stmmac: Introduce selftests support")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>