habanalabs: return old dram bar address upon change

This patch changes the ASIC interface function that changes the DRAM bar
window. The change is to return the old address that the DRAM bar pointed
to instead of an error code.

This simplifies the code that use this function (mainly in debugfs) to
restore the bar to the old setting.

This is also needed for easier support in future ASICs.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>