staging: comedi: ni_tio: convert printk to pr_err

Convert the printk(KERN_ERR ...) in ni_m_clk_src() to a pr_err().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>