Input: ads7846 - add short-hand for spi->dev in probe() function

This will make the code a bit more terse.
No functional change intended.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Link: https://lore.kernel.org/r/20210907200726.2034962-2-daniel@zonque.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>