smbios: only look for a SYSINFO udevice if SYSINFO support is enabled

If SYSINFO support isn't enabled, it's a given that uclass_first_device
for UCLASS_SYSINFO will not find anything, therefore let's skip the test
entirely.

This allows to get rid of the following debug message that may be
confusing:

Cannot find uclass for id 118: please add the UCLASS_DRIVER() declaration for this UCLASS_... id

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>