staging: iio: isl29028: fix incorrect sleep time when taking initial proximity reading

When proximity is enabled in isl29028_enable_proximity(), the function
msleep() is called with the sampling frequency, which is not correct.
This patch changes the code to sleep the specified amount of time listed
in the datasheet instead.

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