iio: gyro: mpu3050: Explicity make a 'poison' value big endian

This clearly has no actual affect but it does show sparse and similar
static analysers that we are doing this intentionally.

CHECK   drivers/iio/gyro/mpu3050-core.c
drivers/iio/gyro/mpu3050-core.c:546:48: warning: incorrect type in assignment (different base types)
drivers/iio/gyro/mpu3050-core.c:546:48:    expected restricted __be16 <noident>
drivers/iio/gyro/mpu3050-core.c:546:48:    got int

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>