mfd: Fix twl-core sparse warning

Fixes below sparse warning.

drivers/mfd/twl-core.c:258:20: warning: symbol 'twl_map' was not declared. Should it be static?
Signed-off-by: G, Manjunath Kondaiah <manjugk@ti.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Tony Lindgren <tony@atomide.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/twl-core.c

@@ -249,7 +249,7 @@ struct twl_mapping {
 	unsigned char sid;	/* Slave ID */
 	unsigned char base;	/* base address */
 };
-struct twl_mapping *twl_map;
+static struct twl_mapping *twl_map;
 
 static struct twl_mapping twl4030_map[TWL4030_MODULE_LAST + 1] = {
 	/*