dmaengine: bcm-scm-raid: statify functions

This driver builds with warnings which can be fixed by making these
functions static.

  CC [M]  drivers/dma/bcm-sba-raid.o
drivers/dma/bcm-sba-raid.c:786:1: warning: no previous prototype for ‘sba_prep_dma_xor_req’ [-Wmissing-prototypes]
 sba_prep_dma_xor_req(struct sba_device *sba,
 ^
drivers/dma/bcm-sba-raid.c:995:1: warning: no previous prototype for ‘sba_prep_dma_pq_req’ [-Wmissing-prototypes]
 sba_prep_dma_pq_req(struct sba_device *sba, dma_addr_t off,
 ^
drivers/dma/bcm-sba-raid.c:1247:1: warning: no previous prototype for ‘sba_prep_dma_pq_single_req’ [-Wmissing-prototypes]
 sba_prep_dma_pq_single_req(struct sba_device *sba, dma_addr_t off,
 ^

Signed-off-by: Vinod Koul <vinod.koul@intel.com>