Bluetooth: Make data pointer of hci_recv_stream_fragment const

The data pointer provided to hci_recv_stream_fragment function should
have been marked const. The function has no business in modifying the
original data. So fix this now.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>