drm/amdkfd: Fix static checker warning on MES queue type

convert_to_mes_queue_type return can be negative, but
queue_input.queue_type is uint32_t. Put return in integer var and cast
to unsigned after negative check.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Graham Sider <Graham.Sider@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>