iio: light: gp2ap020a00f: correct return type to int

iio: light: gp2ap020a00f: correct return type to int

The function gp2ap020a00f_get_thresh_reg() can return -EINVAL in its
error path. Yet, the function has return type of u8. Added error
checking for gp2ap020a00f_get_thresh_reg() return value.

Detected by Smatch:
drivers/iio/light/gp2ap020a00f.c:1013 gp2ap020a00f_get_thresh_reg() warn:
signedness bug returning '(-22)'

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>