ionic: remove unnecessary NULL test

We call ionic_rx_page_alloc() only on existing buf_info structs from
ionic_rx_fill().  There's no need for the additional NULL test.

Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>