env_callback: Mark find_env_callback as static

This is not called outside of env_callback.c so mark static, remove from
<env_callback.h>

Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>