drivers: dma: Mark functions as static in dma_v3.c

Mark the functions ioat3_prep_xor_val(), ioat3_prep_pq_val() and
ioat3_prep_pqxor_val() as static in dma_v3.c because they are not used
outside this file.

This eliminates the following warnings in dma_v3.c:
drivers/dma/ioat/dma_v3.c:741:1: warning: no previous prototype for ‘ioat3_prep_xor_val’ [-Wmissing-prototypes]
drivers/dma/ioat/dma_v3.c:1092:1: warning: no previous prototype for ‘ioat3_prep_pq_val’ [-Wmissing-prototypes]
drivers/dma/ioat/dma_v3.c:1134:1: warning: no previous prototype for ‘ioat3_prep_pqxor_val’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>