sh: dma: Remove unused dmac_search_free_channel()

arch/sh/drivers/dma/dma-api.c:164:5: warning: no previous prototype for 'dmac_search_free_channel' [-Wmissing-prototypes]

dmac_search_free_channel() never had a user in upstream, remove it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/82d5efdde44f9489c5a7d11d0a19750445116c95.1709326528.git.geert+renesas@glider.be


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>