usb: ums: wait for usb cable connection before enter ums mode

Before this change ums mode can not be entered when device
was using the same usb port for usb/uart communication.
Switching USB cable from UART to USB always causes ums exit.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
diff --git a/common/cmd_usb_mass_storage.c b/common/cmd_usb_mass_storage.c
index 99487f4..5f557d5 100644
--- a/common/cmd_usb_mass_storage.c
+++ b/common/cmd_usb_mass_storage.c
@@ -42,6 +42,30 @@
 
 	g_dnl_register("ums");
 
+	/* Timeout unit: seconds */
+	int cable_ready_timeout = UMS_CABLE_READY_TIMEOUT;
+
+	if (!usb_cable_connected()) {
+		puts("Please connect USB cable.\n");
+
+		while (!usb_cable_connected()) {
+			if (ctrlc()) {
+				puts("\rCTRL+C - Operation aborted.\n");
+				goto exit;
+			}
+			if (!cable_ready_timeout) {
+				puts("\rUSB cable not detected.\n" \
+				     "Command exit.\n");
+				goto exit;
+			}
+
+			printf("\rAuto exit in: %.2d s.", cable_ready_timeout);
+			mdelay(1000);
+			cable_ready_timeout--;
+		}
+		puts("\r\n");
+	}
+
 	while (1) {
 		usb_gadget_handle_interrupts();