net: ena: Refactor napi functions

This patch focuses on changes to the XDP part of the napi
polling routine.

1. Update the `napi_comp` stat only when napi is actually
   complete.
2. Simplify the code by using a function pointer to the right
   napi routine (XDP vs non-XDP path)
3. Remove unnecessary local variables.
4. Adjust a debug print to show the processed XDP frame index
   rather than the pointer.

Signed-off-by: David Arinzon <darinzon@amazon.com>
Link: https://lore.kernel.org/r/20240101190855.18739-8-darinzon@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>