i40e: Move i40e_get_head into header file

i40e_get_head needs to be called in multiple files in a further patch,
prepare by moving the function into a header file.

Signed-off-by: Kiran Patil <kiran.patil@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>