Fix panic on sparc64 introduced in my last commit.  I really wish the busdma APIs were more consistent accross architectures.
Fix panic on sparc64 introduced in my last commit.  I really
wish the busdma APIs were more consistent accross architectures.

We should probably move all the other DMA map creations in
xl_attach() where we can really handle them failing, since
xl_init() is void and shouldn't fail.

Pointy hat to:	mux
Tested by:	Anders Andersson <anders@hack.org>

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