efinet: open Simple Network Protocol exclusively

efinet: open Simple Network Protocol exclusively

EDK2 network stack is based on Managed Network Protocol which is layered
on top of Simple Management Protocol and does background polling. This
polling races with grub for received (and probably trasmitted) packets
which causes either serious slowdown or complete failure to load files.

Open SNP device exclusively.  This destroys all child MNP instances and
stops background polling.

Exclusive open cannot be done when enumerating cards, as it would destroy
PXE information we need to autoconfigure interface; and it cannot be done
during autoconfiguration as we need to do it for non-PXE boot as well. So
move SNP open to card ->open method and add matching ->close to clean up.

Based on patch from Mark Salter <msalter@redhat.com>

Also-By: Mark Salter <msalter@redhat.com>
Closes: 41731