net/smsc911x: Use NULL instead of integer for pointer

Silences the following sparse warning:
drivers/net/ethernet/smsc/smsc911x.c:2145:30:
warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>