staging: comedi: change some printk calls to pr_err

There are a handful of calls to printk in ni_stc.h without specified log
levels, as well as one in ni_mio_common.c.  This patch converts these
calls to pr_err() instead, so that they are now explicitly log level
ERR.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>