tools: iio: Send error messages to stderr

This patch indends to make some cleanup and send printf
error messages to stderr. The changes were performed with coccinelle
for failure messages and manual for other cases, such as wrong usage
messages.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>