eth: fbnic: store NAPIs in an array instead of the list

We will need an array for storing NAPIs in the upcoming IRQ handler
reuse rework. Replace the current list we have, so that we are able
to reuse it later.

In a few places replace i as the iterator with t when we iterate
over triads, this seems slightly less confusing than having
i, j, k variables.

Link: https://patch.msgid.link/20241220025241.1522781-7-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>