bcm2835_sdhci: use a macro for interrupts we handle

This is just further simplification, very little functional change. In the
DMA interrupt handler, we *do* now acknowledge both DATA_AVAIL | SPACE_AVAIL
every time -- these operations are mutually exclusive, so while this is a
functional change, it's effectively a nop. Removing the 'mask' local allows
us to further simplify in a future change.