Bluetooth: Add __init and __exit marks to RFCOMM

Those annotation save memory and space on the binary. __init code is
discarded just after execute and __exit code is discarded if the module
is built into the kernel image or unload of modules is not allowed.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>