net: ena: use xdp_return_frame() to free xdp frames

XDP subsystem has a function to free XDP frames and their associated
pages. Using this function would help the driver's XDP implementation to
adjust to new changes in the XDP subsystem in the kernel (e.g.
introduction of XDP MB).

Also, remove 'xdp_rx_page' field from ena_tx_buffer struct since it is
no longer used.

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>