mmc: bfin_sdh: fix dma_desc_array build error

Descriptor array structure has been moved into blackfin dma.h head file.
This patch fix below error:

drivers/mmc/host/bfin_sdh.c:52:8: error: redefinition of 'struct
dma_desc_array'
make[4]: *** [drivers/mmc/host/bfin_sdh.o] Error 1

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>