media: imx: shut up a false positive warning

With imx, gcc produces a false positive warning:

	drivers/staging/media/imx/imx-media-csi.c: In function 'csi_idmac_setup_channel':
	drivers/staging/media/imx/imx-media-csi.c:457:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
	   if (passthrough) {
	      ^
	drivers/staging/media/imx/imx-media-csi.c:464:2: note: here
	  default:
	  ^~~~~~~

That's because the regex it uses for fall trough is not
good enough. So, rearrange the fall through comment in a way
that gcc will recognize.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>