Input: adp5588-keys - get value from data out when dir is out

As discussed here: http://ez.analog.com/message/35852

, the 5587 revC and
5588 revB spec sheets contain a mistake in the GPIO_DAT_STATx register
description.

According to R.Shnell at ADI, as well as my own observations, it should
read: "GPIO data status (shows GPIO state when read for inputs)".

This commit changes the get value function accordingly.

Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>