net: thunderx: introduce a function for mailbox access

This fixes sparse message:

drivers/net/ethernet/cavium/thunder/nicvf_main.c:153:25: sparse: cast to
restricted __le64

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>