iio: chemical: atlas-ph-sensor: use msleep() for long uncritical delays

usleep_range() uses hrtimers and provides no advantage over msleep()
for larger delays.

Cc: Nicholas Mc Guire <hofrat@osadl.org>
Link: http://lkml.org/lkml/2017/1/11/377
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>