i40e: Add bounds check for ch[] array

Add bounds check for ch[] array.
Use ARRAY_SIZE() to ensure that idx is within the range.

Signed-off-by: Piotr Kwapulinski <piotr.kwapulinski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>