Input: ar1021_i2c - use BIT to check for a bit

The MSB for the first byte of touch data transmission is always 1. Make
it a little more obvious we're testing this bit by using BIT(7).

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>