eth: fbnic: support ring channel set while up

Implement the channel count changes. Copy the netdev priv,
allocate new channels using it. Stop, swap, start.
Then free the copy of the priv along with the channels it
holds, which are now the channels that used to be on the
real priv.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://patch.msgid.link/20241220025241.1522781-11-kuba@kernel.org


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