iio: imu: adis16480: assign bias value only if operation succeeded

iio: imu: adis16480: assign bias value only if operation succeeded

This was found only after the whole thing with the inline functions, but
the compiler actually found something. The value of the `bias` (in
adis16480_get_calibbias()) should only be set if the read operation was
successful.

No actual known problem occurs as users of this function all
ultimately check the return value.  Hence probably not stable material.

Fixes: 2f3abe6cbb6c9 ("iio:imu: Add support for the ADIS16480 and similar IMUs")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>