Fix a bug where DMA maps created with bus_dmamap_create() won't increment
the map count and without being able to keep track of the current map
allocation, bus_dma_tag_destroy() will fail to proceed and will return
EBUSY even after all the maps have been correctly destroyed with
bus_dmamap_destroy().

Found while testing the detach method of a NIC.

Tested on:	BBB (am335x)
Reviewed by:	cognet, ian
MFC after:	1 week