iio: magnetometer: yas530: Change data type of hard_offsets to signed

iio: magnetometer: yas530: Change data type of hard_offsets to signed

The "hard_offsets" are currently unsigned u8 but they should be signed as they
can get negative. They are signed in function yas5xx_meaure_offsets() and in the
Yamaha drivers [1][2].

[1] https://github.com/NovaFusion/android_kernel_samsung_golden/blob/cm-12.1/drivers/sensor/compass/yas.h#L156
[2] https://github.com/msm8916-mainline/android_kernel_qcom_msm8916/blob/GT-I9195I/drivers/iio/magnetometer/yas_mag_drv-yas532.c#L91

Fixes: de8860b1ed47 ("iio: magnetometer: Add driver for Yamaha YAS530")
Signed-off-by: Jakob Hauser <jahau@rocketmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/40f052bf6491457d0c5c0ed4c3534dc6fa251c3c.1660337264.git.jahau@rocketmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>