net: ena: Fix using plain integer as NULL pointer in ena_init_napi_in_range

Fix sparse build warning:

drivers/net/ethernet/amazon/ena/ena_netdev.c:2193:34: warning:
 Using plain integer as NULL pointer

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Suggested-by: Joe Perches <joe@perches.com>
Acked-by: Shay Agroskin <shayagr@amazon.com>
Acked-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>