Input: adxl34x - consistently use read/write encapsulation

Don't open code the AC_READ and AC_WRITE macros.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>