nvme: Update nvme_scan_namespace to keep trying on busy

A busy controller shouldn't be game-over for all controllers,
so keep trying on hitting -EBUSY.

This change brings the actual behavior of the routine in line
with what the descriptions says.

Fixes: 982388ea ("nvme: Add NVM Express driver support")
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Moritz Fischer <moritzf@google.com>