net: ena: make symbol 'ena_alloc_map_page' static

The sparse tool complains as follows:

drivers/net/ethernet/amazon/ena/ena_netdev.c:978:13: warning:
 symbol 'ena_alloc_map_page' was not declared. Should it be static?

This symbol is not used outside of ena_netdev.c, so marks it static.

Fixes: 947c54c3

 ("net: ena: Use dev_alloc() in RX buffer allocation")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>