net: stmmac: xgmac: bitrev32 returns u32

The bitrev32 function returns an u32 var, not an int. Fix it.

Fixes: 0efedbf1

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