check-sr-results.yaml: relax fmp check

The firmware management protocol is not required by EBBR, only capsules in
FMP format.
We expect this protocol to be implemented in practice nonetheless, as all
the firmwares we know of have it.

Downgrade the error for missing fmp into a warning.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>