mmc: bcm2835: Drop DMA channel error pointer check

bcm2835_add_host() invokes IS_ERR_OR_NULL() on a DMA channel pointer,
however dma_request_slave_channel() (which was used to populate the
pointer) never returns an error pointer.  So a NULL pointer check is
sufficient.

Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Frank Pavlic <f.pavlic@kunbus.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>