mmc: mmc_spi: Fix mmc_spi_dma_alloc() return type for !HAS_DMA

If CONFIG_NO_DMA=y (e.g. Sun-3 allmodconfig):

    drivers/mmc/host/mmc_spi.c:1323:15: warning: return type defaults to ‘int’ [-Wreturn-type]
     static inline mmc_spi_dma_alloc(struct mmc_spi_host *host) { return 0; }
		   ^~~~~~~~~~~~~~~~~

Fix this by adding the missing return type.

Fixes: a395acf0

 ("mmc: mmc_spi: Allow the driver to be built when CONFIG_HAS_DMA is unset")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20200914094243.3912-1-geert@linux-m68k.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>