dma-bcm2708: Fix module compilation of CONFIG_DMA_BCM2708

dma-bcm2708: Fix module compilation of CONFIG_DMA_BCM2708

bcm2708-dmaengine.c defines functions like bcm_dma_start which are
defined as well in dma-bcm2708.h as inline versions when
CONFIG_DMA_BCM2708 is not defined. This works fine when
CONFIG_DMA_BCM2708 is built in, but when it is selected as module build
fails with redefinition errors because in the build system when
CONFIG_DMA_BCM2708 is selected as module, the macro becomes
CONFIG_DMA_BCM2708_MODULE.

This patch makes the header use CONFIG_DMA_BCM2708_MODULE too when
available.

Fixes https://github.com/raspberrypi/linux/issues/2056

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>