Staging: iio: replace pr_* with dev_*

dev_* is prefered over pr_* when appropriate device stuct is present.
This patch replace pr_err and pr_warn with its dev_ counterpart.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>