dmaengine: use for_each_set_bit

Use for_each_set_bit() to implement for_each_dma_cap_mask() and
remove unused first_dma_cap() and next_dma_cap().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Dan Williams <djbw@fb.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>