net: ena: fix packet's addresses for rx_offset feature

This patch fixes two lines in which the rx_offset received by the device
wasn't taken into account:

- prefetch function:
	In our driver the copied data would reside in
	rx_info->page + rx_headroom + rx_offset

	so the prefetch function is changed accordingly.

- setting page_offset to zero for descriptors > 1:
	for every descriptor but the first, the rx_offset is zero. Hence
	the page_offset value should be set to rx_headroom.

	The previous implementation changed the value of rx_info after
	the descriptor was added to the SKB (essentially providing wrong
	page offset).

Fixes: 68f236df

 ("net: ena: add support for the rx offset feature")
Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>