ethtool: mark ETHER_FLOW as usable for Rx hash

Looks like some drivers (ena, enetc, fbnic.. there's probably more)
consider ETHER_FLOW to be legitimate target for flow hashing.
I'm not sure how intentional that is from the uAPI perspective
vs just an effect of ethtool IOCTL doing minimal input validation.
But Netlink will do strict validation, so we need to decide whether
we allow this use case or not. I don't see a strong reason against
it, and rejecting it would potentially regress a number of drivers.
So update the comments and flow_type_hashable().

Link: https://patch.msgid.link/20250708220640.2738464-4-kuba@kernel.org


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