iio: adc: at91_adc: allow to use full range of startup time

The DT-Property "atmel,adc-startup-time" is stored in an u8 for a microsecond
value. When trying to increase the value of STARTUP in Register AT91_ADC_MR
some higher values can't be reached.

Change the type in function parameter and private structure field from u8 to
u32.

Signed-off-by: Jan Leupold <leupold@rsi-elektrotechnik.de>
[nicolas.ferre@atmel.com: change commit message, increase u16 to u32 for startup time]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>