btrfs: keep const when returning value from get_unaligned_le8()

This was reported by 'gcc -Wcast-qual', the get_unaligned_le8() simply
returns the argument and there's no reason to drop the cast.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>