board: phytec: phytec_som_detection: fix uninitialized bug

When som_type does not match any case, it is uninitialized and the
function still tries to print the SoM info. Rather, this is an error
condition and the function should abort prematurely. Highlight this by
printing an error message and returning early.

Signed-off-by: Yannic Moog <y.moog@phytec.de>