eth: mlx4: Fix IS_ERR() vs NULL check bug in mlx4_en_create_rx_ring

eth: mlx4: Fix IS_ERR() vs NULL check bug in mlx4_en_create_rx_ring

Replace NULL check with IS_ERR() check after calling page_pool_create()
since this function returns error pointers (ERR_PTR).
Using NULL check could lead to invalid pointer dereference.

Fixes: 8533b14b3d65 ("eth: mlx4: create a page pool for Rx")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20250828121858.67639-1-linmq006@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>