Ethtool ring feature has _max_pending attributes read-only.
Set only read-write attributes in nsim_set_ringparam.

This patch is useful, if netdevsim device is set-up using NetworkManager,
because NetworkManager sends 0 as MAX values, as it is pointless to
retrieve them in extra call, because they should be read-only. Then,
the device is left in incosistent state (value > MAX).

Fixes: a7fc6db0 ("netdevsim: support ethtool ring and coalesce settings")
Signed-off-by: Filip Pokryvka <fpokryvk@redhat.com>
Link: https://lore.kernel.org/r/20211210175032.411872-1-fpokryvk@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>