net: stmmac: Do not set RX IC bit if RX Coalesce is zero

We may only want to use the RX Watchdog so lets check if RX Coalesce
settings are non-zero and only set the RX Interrupt on Completion bit if
its not.

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