iio: at91-sama5d2: Limit requested watermark value to hwfifo size

iio: at91-sama5d2: 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: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220117102512.31725-2-paul@crapouillou.net
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>