cam: Move bus_dmamap_load_ccb into cam.c.

This routine is specific to CAM and no longer assumes any internal
bus_dma knowledge as it is simple wrapper around bus_dmamap_load_mem.

Fixes: 60381fd1ee86 memdesc: Retire MEMDESC_CCB.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D41058