dmaengine: bcm2835: Fix polling for completion of DMA with interrupts masked.

The tx_status hook is supposed to be safe to call from interrupt
context, but it wouldn't ever return completion for the last transfer,
meaning you couldn't poll for DMA completion with interrupts masked.

This fixes IRQ handling for bcm2835's DSI1, which requires using the
DMA engine to write its registers due to a bug in the AXI bridge.

Signed-off-by: Eric Anholt <eric@anholt.net>