net: stmmac: xgmac: Reinitialize correctly a variable

'value' was being or'ed with a value from another register. This is a
typo and could cause new written value to be wrong. Fix it.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>