Bluetooth: Add flexible buffer byte order swapping function

Since the SMP code needs to swap ordering of variable length buffers add
a convenience function that can be used for any length buffer.

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

diff --cc net/bluetooth/smp.c
Simple merge