dmaengine: bcm2835: set residue_granularity field

bcm2835-dma supports residue reporting at burst level but didn't report
this via the residue_granularity field.

Without this field set properly we get playback issues with I2S cards.

[by HiassofT, taken from bcm2708-dmaengine]
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>