spi: omap3: Make local functions as static

Attach static on local defined functions.

Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Jagan Teki <jteki@openedev.com>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>