Fix pagelist bus_dmamap_t map handling. Memory for pagelist is allocated
by bus_dmamem_alloc() which creates associated bus_dmamap_t map for us.
When this memory is freed by bus_dmamem_free(), the map is freed as well.

Thus there is no need to free it explicitly by bus_dmamap_destroy(),
which leads to double freeing.

Discussed with: gonzo
Approved by: kib (mentor)