net: lan966x: improve error handle in lan966x_fdma_rx_get_frame()

Don't just print a warning.  Clean up and return an error as well.

Fixes: c8349639

 ("net: lan966x: Add FDMA functionality")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/YwjgDm/SVd5c1tQU@kili
Signed-off-by: Jakub Kicinski <kuba@kernel.org>