iio: hid-sensor-prox: Fix return values

IIO_CHAN_INFO_SAMP_FREQ and IIO_CHAN_INFO_HYSTERESIS cases ignored
the actual return values (which could be -EINVAL) and instead
returned IIO_VAL_INT_PLUS_MICRO always. Return the actual value
obtained from the functions. Both functions return IIO_VAL_INT_PLUS_MICRO
upon success.

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>