auxdisplay: ht16k33: Add helper variable dev

This driver has many users of "client->dev".  Add shorthands to simplify
the code.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>