iio: ssp_sensors: fix error code in ssp_print_mcu_debug()

The ssp_print_mcu_debug() function should return negative error codes on
error.  Returning "length" is meaningless.  This change does not affect
runtime because the callers only care about zero/non-zero.

Reported-by: Jonathan Cameron <jic23@kernel.org>
Fixes: 50dd64d5

 ("iio: common: ssp_sensors: Add sensorhub driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210914105333.GA11657@kili
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>