net: thunderx: add static

This fixes sparse messages like this:

drivers/net/ethernet/cavium/thunder/nicvf_main.c:1141:26: sparse: symbol
'nicvf_get_stats64' was not declared. Should it be static?

Also remove unused declarations

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Aleksey Makarov <aleksey.makarov@caviumnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>