eth: fbnic: support ring channel get and set while down

Trivial implementation of ethtool channel get and set. Set is only
supported when device is closed, next patch will add code for
live reconfig.

Asymmetric configurations are supported (combined + extra Tx or Rx),
so are configurations with independent IRQs for Rx and Tx.
Having all 3 NAPI types (combined, Tx, Rx) is not supported.

We used to only call fbnic_reset_indir_tbl() during init.
Now that we call it after device had been register must
be careful not to override user config.

Link: https://patch.msgid.link/20241220025241.1522781-10-kuba@kernel.org


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