serial: pxa: work around for errata #20

Errata E20: UART: Character Timeout interrupt remains set under certain
software conditions.

Implication: The software servicing the UART can be trapped in an infinite loop.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>