Input: ads7846 - convert to full duplex

Input: ads7846 - convert to full duplex

Starting with ba3f62d8cb98 ("Input: ads7846 - extend the driver for ads7845
controller support"), the ads7845 was partially converted to full duplex
mode.

Since it is not touchscreen controller specific, it is better to extend
this conversion to cover entire driver. This will reduce CPU load and make
driver more readable.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20201110085041.16303-2-o.rempel@pengutronix.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>