fw-c/test.c: use appropriate type for variable
fw-c/test.c: use appropriate type for variable

python maps it as uint32_t, and signed types should be avoided whenever
possible on PRU since they are not natively supported.

mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio