btrfs: drop unnecessary variable in btrfs_init_new_device

There is only usage of the declared devices variable, instead use its
value directly.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>