tty/serial: atmel: manage shutdown in case of RS485 or ISO7816 mode

In atmel_shutdown() we call atmel_stop_rx() and atmel_stop_tx() functions.
Prevent the rx restart that is implemented in RS485 or ISO7816 modes when
calling atmel_stop_tx() by using the atomic information tasklet_shutdown
that is already in place for this purpose.

Fixes: 98f2082c

 ("tty/serial: atmel: enforce tasklet init and termination sequences")
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200210152053.8289-1-nicolas.ferre@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>