bluetooth: fix warning in net/bluetooth/rfcomm/sock.c

fix this warning:

  net/bluetooth/rfcomm/sock.c: In function ‘rfcomm_sock_ioctl’:
  net/bluetooth/rfcomm/sock.c:795: warning: unused variable ‘sk’

perhaps BT_DEBUG() should be improved to do printf format checking
instead of the #ifdef, but that looks quite intrusive: each bluetooth
.c file undefines the macro.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>