usb: Tidy up the usb_start flag

This should be declared in a header file so that type-checking works
correctly.

Add a single declaration to usb.h and remove the others.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/common/usb.c b/common/usb.c
index ae9253d..836506d 100644
--- a/common/usb.c
+++ b/common/usb.c
@@ -43,7 +43,7 @@
 #define USB_BUFSIZ	512
 
 static int asynch_allowed;
-char usb_started; /* flag for the started/stopped USB status */
+bool usb_started; /* flag for the started/stopped USB status */
 
 #if !CONFIG_IS_ENABLED(DM_USB)
 static struct usb_device usb_dev[USB_MAX_DEVICE];