mtd: hyperbus: hbmc-am654: Fix direct mapping setup flash access

Setting up of direct mapping should be done with flash node's IO
address space and not with controller's IO region.

Fixes: b6fe8bc6 ("mtd: hyperbus: move direct mapping setup to AM654 HBMC driver")
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20200924081214.16934-3-vigneshr@ti.com