Input: ads7846 - convert to full duplex

Starting with 3eac5c7e ("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>