staging: comedi: remove comedi_error()

The comedi_error() function is just a wrapper around dev_err() that adds
the dev->driver->driver_name prefix to the message and a terminating
new-line character.

All of the users of this function have been converted to use dev_err()
directly. Remove the now unused function.

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