net: libwx: fix an error code in wx_alloc_page_pool()

This function always returns success.  We need to preserve the error
code before setting rx_ring->page_pool = NULL.

Fixes: 850b9711

 ("net: libwx: Allocate Rx and Tx resources")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/Y+T4aoefc1XWvGYb@kili
Signed-off-by: Jakub Kicinski <kuba@kernel.org>