i40e: Fix uninitialized variable

We have an uninitialized variable warning for valid_len for one case in
validate_vf_mesg. To fix this, just initialize it to 0 at the top of the
function and remove all of the now redundant assignments to 0 in the
individual cases.

Change-Id: Iacbd97f4c521ed8d662eef803a598d8707708cfd
Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>