iio: stmpe-adc: Enable all stmpe-adc interrupts just once

This commit will enable the interrupts of all channels handled by this
driver only once in the probe function.

This will improve performance because one byte less has to be written over
i2c on each read out of the adc. On the fastest ADC mode this will improve
read out speed by 15%.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>