Fixed prod_to_dev_downgrade_allowed logic in validate_transition
Fixed prod_to_dev_downgrade_allowed logic in validate_transition

The documentation for the prod_to_dev_downgrade_allowed function pointer
reads, "If NULL, treated as if the function always returns false."

Return `LIBCR51SIGN_ERROR_DEV_DOWNGRADE_DISALLOWED` if prod_to_dev
allowed return false or is NULL.

Change-Id: I4750256d308096e706bb9e0e0266d365b6f5a026
Signed-off-by: Willy Tu <1e3df9bf0ac020872e0e39905eae1676fb455af4@google.com>

openbmc/google-misc,openbmc/google-misc,openbmc/google-misc