dmaengine: ste_dma40: Correct copy/paste error

'struct stedma40_half_channel_info's header comment says that it's
called 'struct stedma40_chan_cfg'. Let's straighten that out.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>