bus_get_dma_tag now necessary for drivers using bus_dma

[M. Warner Losh]

In message: <14026.1157478288 at critter.freebsd.dk>
            "Poul-Henning Kamp" <phk at phk.freebsd.dk> writes:
: In message <20060905173334.GH9421 at funkthat.com>, John-Mark Gurney writes:
: 
: >This means that for each call to bus_dma_tag_create, instead of passing
: >a NULL pointer, you should call bus_get_dma_tag(yourdev) and use that
: >as the parent tag.  I committed some example code to various drivers,
: >such as ahc, ata, em, and ohci.
: 
: If there are never any exeptions to this requirement, why not
: pass "yourdev" and have the magic hidden, rather than add 17
: ritual characters to the API ?

As Scott pointed out, it won't always be NULL.  I like his idea of
having a variant that takes a device_t...

Warner