net: ena: Make some functions static

Fix sparse warnings:

drivers/net/ethernet/amazon/ena/ena_netdev.c:460:6: warning: symbol 'ena_xdp_exchange_program_rx_in_range' was not declared. Should it be static?
drivers/net/ethernet/amazon/ena/ena_netdev.c:481:6: warning: symbol 'ena_xdp_exchange_program' was not declared. Should it be static?
drivers/net/ethernet/amazon/ena/ena_netdev.c:1555:5: warning: symbol 'ena_xdp_handle_buff' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>