gdrom: fix mistake in assignment of error

Apparently the kbuild bots missed this one originally, but at least
it caught it now. Fix compilation error:

All errors (new ones prefixed by >>):

drivers/cdrom/gdrom.c: In function 'probe_gdrom':
>> drivers/cdrom/gdrom.c:797:3: error: 'rc' undeclared (first use in this function)
rc = PTR_ERR(gd.gdrom_rq);
^~
drivers/cdrom/gdrom.c:797:3: note: each undeclared identifier is reported only once for each function it appears in

Fixes: ad5fc6bb7221 ("gdrom: convert to blk-mq")
Signed-off-by: Jens Axboe <[e-mail]>

drivers/cdrom/gdrom.c

···
 	gd.gdrom_rq = blk_mq_init_sq_queue(&gd.tag_set, &gdrom_mq_ops, 1,
 				BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_BLOCKING);
 	if (IS_ERR(gd.gdrom_rq)) {
-		rc = PTR_ERR(gd.gdrom_rq);
+		err = PTR_ERR(gd.gdrom_rq);
 		gd.gdrom_rq = NULL;
 		goto probe_fail_requestq;
 	}