Bluetooth: Fix context in RFCOMM tty

We now run in process context, no need to disable interrupts.
Calls from the tty layer also run in process context.

rw_lock was converted to spinlock, we have more writers than readers in
this case.

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