[BLUETOOTH] rfcomm_worker(): fix wakeup race

Set TASK_INTERRUPTIBLE prior to testing the flag to avoid missed wakeups.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>