staging: iio: tsl2x7x: use usleep_range() instead of mdelay()

This driver in some cases can busy wait for upwards of 15ms. Since the
kernel at this point is not running in atomic context, and is running in
process context, we can safely use usleep_range() instead. This patch
changes the two occurrences of mdelay() to usleep_range().

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>