Bluetooth: Rename __rfcomm_dev_get() to __rfcomm_dev_lookup()

Functions which search lists for matching id's are more
commonly named *_lookup, which is the convention in the
bluetooth core as well.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Tested-By: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>