HID: hid-sensor-custom: Remove unused variable 'ret'

Fixes the following W=1 kernel build warning(s):

 drivers/hid/hid-sensor-custom.c: In function ‘store_value’:
 drivers/hid/hid-sensor-custom.c:400:7: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

Cc: Jiri Kosina <jikos@kernel.org>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-input@vger.kernel.org
Cc: linux-iio@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>