staging: iio: isl29028: change mdelay() to msleep()

This driver in some cases can busy wait for upwards of 100 ms. Since the
kernel at this point is not running in atomic context, and is running in
process context, we can safely use msleep() instead. This patch changes
the two occurrences of mdelay() to msleep().

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>