net: stmmac: selftests: Flow Control test can also run with ASYM Pause

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>