gpu: ipu-v3: ipu-dmfc: Make function ipu_dmfc_init_channel() return void

Since the function ipu_dmfc_init_channel() always returns zero, we may
change the return type to void to simplify the code.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>