iio: light: apds9960: fix wrong register for gesture gain

commit 0aa60ff5 upstream.

Gesture Gain Control is in REG_GCONF_2 (0xa3), not in REG_CONFIG_2 (0x90).

Fixes: aff268cd

 ("iio: light: add APDS9960 ALS + promixity driver")
Signed-off-by: Alejandro Concepcion-Rodriguez <asconcepcion@acoro.eu>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/EaT-NKC-H4DNX5z4Lg9B6IWPD5TrTrYBr5DYB784wfDKQkTmzPXkoYqyUOrOgJH-xvTsEkFLcVkeAPZRUODEFI5dGziaWXwjpfBNLeNGfNc=@acoro.eu
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>