twl4030-charger: Fix compiler warning with regulator_enable()

The return value of regulator_enable need to be checked. This patch fixes
the following warning:
drivers/power/twl4030_charger.c: In function ‘twl4030_charger_enable_usb’:
drivers/power/twl4030_charger.c:192:20: warning: ignoring return value of ‘regulator_enable’, declared with attribute warn_unused_result [-Wunused-result]
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>

drivers/power/twl4030_charger.c

@@ -189,7 +189,12 @@ static int twl4030_charger_enable_usb(struct twl4030_bci *bci, bool enable)
 
 		/* Need to keep regulator on */
 		if (!bci->usb_enabled) {
-			regulator_enable(bci->usb_reg);
+			ret = regulator_enable(bci->usb_reg);
+			if (ret) {
+				dev_err(bci->dev,
+					"Failed to enable regulator\n");
+				return ret;
+			}
 			bci->usb_enabled = 1;
 		}