i40e: Make local function i40e_get_link_speed static

Fixes the following sparse warning:

drivers/net/ethernet/intel/i40e/i40e_main.c:5440:5: warning:
 symbol 'i40e_get_link_speed' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>