tty: isicom: fix deadlock on shutdown

Alexander Strakh <strakh@ispras.ru> reported

	KERNEL_VERSION: 2.6.31
	DESCRIBE:
Driver drivers/char/isicom.c might sleep in atomic  context, because it
calls 
tty_port_xmit_buf under spin_lock.

./drivers/char/isicom.c:
1307 static void isicom_hangup(struct tty_struct *tty)
1308 {
...
1315         spin_lock_irqsave(&port->card->card_lock, flags);
1316         isicom_shutdown_port(port);
...

Path to might_sleep macro from isicom_hangup:
1. isicom_hangup calls spin_lock_irqsave (drivers/char/isicom.c:1315) and
then 
calls isicom_shutdown_port.
2. isiscom_shutdown_port calls tty_port_free_xmit_buf at 
drivers/char/isicom.c:906
3. tty_port_free_xmit_buf calls mutex_lock at drivers/char/tty_port:48

Found by Linux Driver Verification Project.

Reported-by: Alexander Strakh <strakh@ispras.ru>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>