sync_page() was conditionalizing it's whole fn body on the bdrv being
non-null. Just return for the function immediately on NULL brdv and
get rid of the big if.

Makes implementation consistent with flash_zynq_area().
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>