[PATCH] media: imx: work around false-positive warning, again

A warning that I thought to be solved by a previous patch of mine has resurfaced with gcc-8: media/imx/imx-media-csi.c: In function 'csi_link_validate': media/imx/imx-media-csi.c:1025:20: error: 'upstream_ep' may be used uninitialized in this function [-Werror=maybe-uninitialized] media/imx/imx-m

[PATCH] media: imx: work around false-positive warning

The IS_ERR()/PTR_ERR() combination confuses gcc to the point that it cannot prove the upstream_ep variable to be initialized: drivers/staging/media/imx/imx-media-csi.c: In function 'csi_link_validate': drivers/staging/media/imx/imx-media-csi.c:1025:20: error: 'upstream_ep' may be used uninitializ