iio: ti_am335x_adc: check if we found the value

Usually we get all the values we wanted but it is possible, that te ADC
unit is busy performing the conversation for the HW events. In that case
-EBUSY is returned and the user may re-call the function.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>