tty/serial: at91: Handle shutdown more safely

Interrupts were being cleaned up late in the shutdown handler, it is possible
that an interrupt can occur and schedule a tasklet that runs after the port is
cleaned up. There is a null dereference due to this race condition with the
following stacktrace:

[<c02092b0>] (atmel_tasklet_func+0x514/0x814) from [<c001fd34>] (tasklet_action+0x70/0xa8)
[<c001fd34>] (tasklet_action+0x70/0xa8) from [<c001f60c>] (__do_softirq+0x90/0x144)
[<c001f60c>] (__do_softirq+0x90/0x144) from [<c001fa18>] (irq_exit+0x40/0x4c)
[<c001fa18>] (irq_exit+0x40/0x4c) from [<c000e298>] (handle_IRQ+0x64/0x84)
[<c000e298>] (handle_IRQ+0x64/0x84) from [<c000d6c0>] (__irq_svc+0x40/0x50)
[<c000d6c0>] (__irq_svc+0x40/0x50) from [<c0208060>] (atmel_rx_dma_release+0x88/0xb8)
[<c0208060>] (atmel_rx_dma_release+0x88/0xb8) from [<c0209740>] (atmel_shutdown+0x104/0x160)
[<c0209740>] (atmel_shutdown+0x104/0x160) from [<c0205e8c>] (uart_port_shutdown+0x2c/0x38)

Signed-off-by: Marek Roszko <mark.roszko@gmail.com>
Acked-by: Leilei Zhao <leilei.zhao@atmel.com>
Cc: <stable@vger.kernel.org> # v3.12
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>