eth: fbnic: split fbnic_disable()

Factor out handling a single nv from fbnic_disable() to make
it reusable for queue ops. Use a __ prefix for the factored
out code. The real fbnic_nv_disable() which will include
fbnic_wrfl() will be added with the qops, to avoid unused
function warnings.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250901211214.1027927-7-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>