cdrom: cleanup hardcoded error-code

This patch eliminates hardcoded return value of register_cdrom().

It also changes the return value to -EINVAL.
It is more appropriate than -2 (-ENOENT) because it is only
happen invalid usage of register_cdrom() by broken cdrom driver.
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/cdrom/cdrom.c

@@ -393,7 +393,7 @@ int register_cdrom(struct cdrom_device_info *cdi)
 	cdinfo(CD_OPEN, "entering register_cdrom\n"); 
 
 	if (cdo->open == NULL || cdo->release == NULL)
-		return -2;
+		return -EINVAL;
 	if (!banner_printed) {
 		printk(KERN_INFO "Uniform CD-ROM driver " REVISION "\n");
 		banner_printed = 1;