[efi] Assume that vendor wireless drivers are unusable via SNP

[efi] Assume that vendor wireless drivers are unusable via SNP

The UEFI model for wireless network boot cannot sensibly be described
without cursing.  Commit 758a504 ("[efi] Inhibit calls to Shutdown()
for wireless SNP devices") attempts to work around some of the known
issues.

Experimentation shows that on at least some platforms (observed with a
Lenovo ThinkPad T14s Gen 5) the vendor SNP driver is broken to the
point of being unusable in anything other than the single use case
envisioned by the firwmare authors.  Doing almost anything directly
via the SNP protocol interface has a greater than 50% chance of
locking up the system.

Assume, in the absence of any evidence to the contrary so far, that
vendor SNP drivers for wireless network devices are so badly written
as to be unusable.  Refuse to even attempt to interact with these
drivers via the SNP or NII protocol interfaces.

Signed-off-by: Michael Brown <mcb30@ipxe.org>