bcm2835-mmc/sdhost: of_dma_request_slave_channel isn't exported...

of_dma_request_slave_channel isn't an exported function, so can't be
used from within a module. Replace with dma_request_slave_channel,
which calls the of_ variant but IS exported.

See: https://github.com/raspberrypi/linux/issues/952



Signed-off-by: Phil Elwell <phil@raspberrypi.org>