tty/serial: at91: prevent null dereference in tasklet function

Something asks a tasklet to be scheduled when the uart port is closed.
Need to supress the kernel panic for now by checking if the port is NULL or
not.

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>