nodedev: fix build with clang

nodedev: fix build with clang

Build fails with:

conf/node_device_conf.c:825:62: error: comparison of unsigned enum expression < 0 is always false [-Werror,-Wtautological-compare]
    if ((data->drm.type = virNodeDevDRMTypeFromString(type)) < 0) {
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~
conf/node_device_conf.c:1801:59: error: comparison of unsigned enum expression < 0 is always false [-Werror,-Wtautological-compare]
        if ((type = virNodeDevDevnodeTypeFromString(tmp)) < 0) {
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~
2 errors generated.

Fix by using intermediate variable to store the result similarly
to how it's done for other FromString* calls.