tsc2301 - correct the handling of the DAV interrupt

The tsc2301 is not like the ads7846! The tsc2301 does
all converting, timing and filtering autonomously.

The chip works by measuring each channel multiple times and
the average of those values is stored in the corresponding
register before it processes the next channel.

Reading the X,Y,Z1,Z2 registers asynchronously to DAV interrupt
results in subtle errors.

For example instead of drawing (fast) diagonal lines you will
draw "staircases".

Signed-off-by: Klaus Pedersen <klaus.k.pedersen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>