dmaengine: bcm2708: Remove chancnt affectations

Mirror bcm2835-dma.c commit 9eba5536:
chancnt is already filled by dma_async_device_register, which uses the channel
list to know how much channels there is.

Since it's already filled, we can safely remove it from the drivers' probe
function.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>