OMAP4: fix omap4_bootsource regression

commit d7a913469c34553e96c887f8a9934bacd794e81c breaks boot source detection
for pcm049. The tracing vectors shows all tested boot sources, so order is
important. By not returning but overwriting src we effectively reversed the
order if more than one flag is set.

Signed-off-by: Jan Weitzel <j.weitzel@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>