ixgbe: use ARRAY_SIZE for array sizing calculation on array buf

Use the ARRAY_SIZE macro on array buf to determine size of the array.
Improvement suggested by coccinelle.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>