bus/dpaa: do nothing if bus not present

bus/dpaa: do nothing if bus not present

The DPAA bus support code put out messages like:
	dpaax: read_memory_node():	 Unable to glob device-tree memory node:
		(/proc/device-tree/memory[@0-9]*/reg)(3)
	dpaax: PA->VA translation not available;
	dpaax: Expect performance impact.

These are unnecessary and likely to confuse the end user.
Fix this by doing nothing if bus is empty.

Fixes: 5a7dbb934d75 ("dpaa: enable dpaax library")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Shreyansh Jain <shreyansh.jain@nxp.com>