net: stmmac: Refactor __stmmac_xdp_run_prog for XDP ZC

Prepare stmmac_xdp_run_prog() for AF_XDP zero-copy support which will be
added by upcoming patches by splitting out the XDP verdict processing
into __stmmac_xdp_run_prog() and it callable for XDP ZC path which does
not need to verify bpf_prog is not NULL.

The stmmac_xdp_run_prog() is used for regular XDP Rx path which requires
bpf_prog to be verified.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>