video: xilinxfb: Rename PLB_ACCESS_FLAG to BUS_ACCESS_FLAG

Using only PLB name is wrong for a long time because
the same access functions are also used for AXI.
s/PLB/BUS/g

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>