net: stmmac: refactor stmmac_init_rx_buffers for stmmac_reinit_rx_buffers

The per-queue RX buffer allocation in stmmac_reinit_rx_buffers() can be
made to use stmmac_alloc_rx_buffers() by merging the page_pool alloc
checks for "buf->page" and "buf->sec_page" in stmmac_init_rx_buffers().

This is in preparation for XSK pool allocation later.
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>