iio: Add iio_device_get()

This patch add the iio_device_get() function, which increases the reference
count of a iio device. The matching function to decrease the reference count -
iio_device_put() - already exists.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>