net: ena: cosmetic: minor code changes

1. Use BIT macro instead of shift operator for code clarity
2. Replace multiple flag assignments to a single assignment of multiple
   flags in ena_com_add_single_rx_desc()
3. Move ENA_HASH_KEY_SIZE from ena_netdev.h to ena_com.h

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