[SCSI] sym53c8xx: Simplify DAC DMA handling

By introducing the use_dac(), set_dac() and DMA_DAC_MASK macros, we can
eliminate a lot of ifdefs from the code.  We now rely on the compiler to
optimise away a few things that we'd formerly relied on the preprocessor
to do.  This makes sym_setup_bus_dma_mask() small enough to inline into
its only caller.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>