staging: most: cdev: add missing check for cdev_add failure

Currently the call to cdev_add is missing a check for failure. Fix this by
checking for failure and exiting via a new error path that ensures the
allocated comp_channel struct is kfree'd.

Detected by CoverityScan, CID#1462359 ("Unchecked return value")

Fixes: 9bc79bbc ("Staging: most: add MOST driver's aim-cdev module")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>