atmel_serial: fix spinlock lockup in RS485 code

commit dbf1115d3f8c7052788aa4e6e46abd27f3b3eeba upstream.

Patch to fix a spinlock lockup in the driver that sometimes happens when the
tasklet starts.

Signed-off-by: Claudio Scordino <claudio@evidence.eu.com>
Signed-off-by: Dave Bender <codehero@gmail.com>
Tested-by: Dave Bender <codehero@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>