spi: bcm2835: added comment about different bus behaviour of DMA mode

The DMA mode behaves slightly different than polling or interrupt driven
mode, so just document the fact

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>

Changelog:
  V1 -> V2: applied feedback by Stefan Wahren
            new in V2
Signed-off-by: Mark Brown <broonie@kernel.org>