cnss: Fix spelling and downgrade error level

Fix spelling in error message; also downgrade the error level.
Swappable partition is an optional feature, not finding the image
is not an error.

CRs-Fixed: 732663
Change-Id: If3c15d6ae1984d75db6ca7e64766eff3198e8bbe
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>