gve: Refactor napi add and remove functions

This change makes the napi poll functions non-static and moves the
gve_(add|remove)_napi functions to gve_utils.c, to make possible future
"start queue" hooks in the datapath files.

Signed-off-by: Shailend Chand <shailend@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Jeroen de Borst <jeroendb@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240122182632.1102721-3-shailend@google.com


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