tty/serial: at91: fix I/O accesses on RHR and THR for AVR32

This patch fixes I/O accesses on the Receiver Holding Register and on the
Transmitter Holding Register. Indeed AVR32 can only perform 32bit I/O
accesses on registers: using 8bit I/O accesses would read or write garbage
data.

Fixes: commit b5199d46 ("tty/serial: at91: add support to FIFOs")
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>