media: omap3isp: mark isp_isr_dbg as __maybe_unused

This function is only used for debugging purposes. When DEBUG
is disabled, it becomes unused, causing a clang warning with W=1.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>