Bluetooth: Move rfcomm_get_device() before rfcomm_dev_activate()

This is a preparatory patch which moves the rfcomm_get_device()
definition before rfcomm_dev_activate() where it will be used.

Signed-off-by: Gianluca Anzolin <gianluca@sottospazio.it>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>