iio: adc: ina219: Avoid underflow for sleeping time

Proper support for the INA219 lowered the minimum sampling period from
2*140us to 2*84us. Subtracting 200us later leads to an underflow and
an almost infinite udelay later.

Using a signed int for the sampling period provides sufficient range
(at most 2*8640*1024us), but catches the underflow when comparing with
buffer_us.

Fixes: 18edac2e ("iio: adc: Fix integration time/averaging for INA219/220")
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>