net: ena: aggregate doorbell common operations into a function

The ena_ring_tx_doorbell() is introduced to call the doorbell and
increase the driver's corresponding stat.

Signed-off-by: Ido Segev <idose@amazon.com>
Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>