usb: dwc3: omap: fix sparse warning

our global '_omap' pointer wasn't marked
static. This patch solves the following sparse
warning:

	warning: symbol '_omap' was not declared. \
	Should it be static?
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/dwc3-omap.c

@@ -126,7 +126,7 @@ struct dwc3_omap {
 	u32			dma_status:1;
 };
 
-struct dwc3_omap		*_omap;
+static struct dwc3_omap		*_omap;
 
 static inline u32 dwc3_omap_readl(void __iomem *base, u32 offset)
 {