hw/arm/omap_gpmc: Avoid buffer overrun filling prefetch FIFO

In fill_prefetch_fifo(), if the device we are reading from is 16 bit,
then we must not try to transfer an odd number of bytes into the FIFO.
This could otherwise have resulted in our overrunning the prefetch.fifo
array by one byte.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
