Input: edt-ft5x06 - fix setting gain, offset, and threshold via device tree

A recent patch broke parsing the gain, offset, and threshold parameters
from device tree. Instead of setting the cached values and writing them
to the correct registers during probe, it would write the values from DT
into the register address variables and never write them to the chip
during normal operation.

Fixes: 2e23b7a9 ("Input: edt-ft5x06 - use generic properties API")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>