staging: comedi: fix clean-up of comedi_class in comedi_init()

There is a clean-up bug in the core comedi module initialization
functions, `comedi_init()`.  If the `comedi_num_legacy_minors` module
parameter is non-zero (and valid), it creates that many "legacy" devices
and registers them in SysFS.  A failure causes the function to clean up
and return an error.  Unfortunately, it fails to destroy the "comedi"
class that was created earlier.  Fix it by adding a call to
`class_destroy(comedi_class)` at the appropriate place in the clean-up
sequence.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -2915,6 +2915,7 @@ static int __init comedi_init(void)
 		dev = comedi_alloc_board_minor(NULL);
 		if (IS_ERR(dev)) {
 			comedi_cleanup_board_minors();
+			class_destroy(comedi_class);
 			cdev_del(&comedi_cdev);
 			unregister_chrdev_region(MKDEV(COMEDI_MAJOR, 0),
 						 COMEDI_NUM_MINORS);