ice: Make ice_msix_clean_rings static

commit 158a08a6

 ("ice: remove ndo_poll_controller") removed
ice_netpoll and introduced a namespace warning for ice_msix_clean_rings.
Fix the namespace warning by making ice_msix_clean_rings static.

Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>