dmaengine: bcm2835: Fix position reporting for 40 bits channels

For 40 bits channels, the position is reported by reading the upper byte
in the SRCI/DESTI registers. However the driver adds that upper byte
with an 8-bits left shift, while it should be 32.

Fixes: 9a52a991 ("bcm2835-dma: Add proper 40-bit DMA support")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>