libmbim,uuid: fix build with -Werror=type-limits
mbim-uuid.c: In function 'mbim_uuid_from_service':
mbim-uuid.c:400:5: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
mbim-uuid.c: In function 'mbim_uuid_from_context_type':
mbim-uuid.c:594:5: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
Edited by Aleksander Morgado