Don't entagle the code paths for the two NIC classes if it's not necessary.

Only compile tested.

Change-Id: I59164ccf54afbbd64a0598282d13e80ff7fd6fa4
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/33637


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>