mvpp2: fix pointer check

priv->page_pool is an array, so comparing against it will always return true.
Do a meaningful check by checking priv->page_pool[0] instead.
While at it, clear the page_pool pointers on deallocation, or when an
allocation error happens during init.

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: c2d6fe61 ("mvpp2: XDP TX support")
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>