arm64: mm: improve the mem-offline device node availability check

Even if we use status = "disabled" in mem-offline node, still the
update_memory_limit() will go and update memory_limit variable and
thus the removed memory is never added to system as the
mem-offline driver is disabled. Correct this logic.

Change-Id: I80102830f0a5a82dcd4cddfaddb0de21bfdd0110
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>