dtoc: Make integer division python 3.x safe

dtoc: Make integer division python 3.x safe

If we use the '/' operator then python 3.x will produce a float, and
refuse to multiply the string sequence in Conv_name_to_c by it with:

    TypeError: can't multiply sequence by non-int of type 'float'

Use the '//' operator instead to enforce that we want integer rather
than floating point division.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Simon Glass <sjg@chromium.org>