net: ena: minor performance improvement

Reduce fastpath overhead by making ena_com_tx_comp_req_id_get() inline.
Also move it to ena_eth_com.h file with its dependency function
ena_com_cq_inc_head().

Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>