thunderx: enable page recycling for non-XDP case

Commit 77322538 ("net: thunderx: Optimize page recycling for XDP")
added code to nicvf_alloc_page() that inadvertently disables receive buffer
page recycling for the non-XDP case by always NULL'ng the page pointer.

This patch corrects two if-conditionals to allow for the recycling of non-XDP
mode pages by only setting the page pointer to NULL when the page is not ready
for recycling.

Fixes: 77322538

 ("net: thunderx: Optimize page recycling for XDP")
Signed-off-by: Dean Nelson <dnelson@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>