[media] staging: media: omap4iss: mark omap4iss_flush() static
We get 1 warning when building kernel with W=1:
drivers/staging/media/omap4iss/iss.c:64:6: warning: no previous prototype for 'omap4iss_flush' [-Wmissing-prototypes]
In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.