staging: comedi: change device used in dev_...() calls

A previous set of patches by Ravishankar Karkala Mallikarjunayya
replaced a load of printk() calls with dev_info(), dev_err(), etc.
Unfortunately, these used the 'struct device *hw_dev' member of 'struct
comedi_device') as the first parameter of these dev_...() calls, but
that pointer is usually NULL, so the kernel log messages come out a bit
wrong (they contain the phrase "(NULL device *)").

Use the 'struct device *class_dev' member of 'struct comedi_device'
instead for these dev_...() calls.  It will be non-NULL and somewhat
meaningful to users.  It's also consistent with those comedi drivers
that already use the class_dev member in their dev_...() calls.

Some of the messages included the format "comedi%d" with the minor
device number used for the "%d".  This is now redundant as it will be
the same as the dev_name() part of the kernel log message produced by
the dev_...() calls.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>