net: lan966x: cleanup error handling in lan966x_fdma_rx_alloc_page_pool()

net: lan966x: cleanup error handling in lan966x_fdma_rx_alloc_page_pool()

This code works, but there are a few things to tidy up:
1. No need to an unlikely() because IS_ERR() already has an unlikely()
   built in.
2. No need to use PTR_ERR_OR_ZERO() because it's not an error pointer.
3. Use the returned error code directly instead of using groveling in
   rx->page_pool to find it.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://patch.msgid.link/ag7_YBWRpRmY9MGT@stanley.mountain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>