[PARISC] Make DMA routines more stubby

We were pretending to use the GENERIC_ISA_DMA routines, but never
selected that symbol.  Since ISA DMA is known to not work right now,
just remove the attempts to acquire the dma_spin_lock to fix compile
warnings.

Signed-off-by: Matthew Wilcox <willy@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>