iio: imu: st_lsm6dsx: Limit requested watermark value to hwfifo size

Instead of returning an error if the watermark value is too high, which
the core will silently ignore anyway, limit the value to the hardware
FIFO size; a lower-than-requested value is still better than using the
default, which is usually 1.

Cc: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/20220117102512.31725-1-paul@crapouillou.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>