r600/sfn: Use correct setter method.
Fix warning reported by Coverity Scan.
Useless call (USELESS_CALL)
side_effect_free: Calling v->pin_to_channel() is only useful for its
return value, which is ignored.
Fixes: 5d10e3ec6066 ("r600/nir: Pin interpolation results to channel")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
cc: @gerddie