mfd: stmpe: Fix STMPE24xx GPMR LSB

The least significat byte of the GPIO value read register
on the STMPE24xx series is on addres 0xA4 not 0xA5. Correct
against datasheet and tested on the STMPE2401 hardware.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>