USB: fix build with XEN and EARLY_PRINTK_DBGP enabled but USB_SUPPORT disabled

Since there's no possible caller of dbgp_external_startup() and
dbgp_reset_prep() when !USB_EHCI_HCD, there's no point in building and
exporting these functions in that case. This eliminates a build error
under the conditions listed in the subject, introduced with the merge
f1c6872e4980bc4078cfaead05f892b3d78dea64.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/early/ehci-dbgp.c

@@ -20,6 +20,7 @@
 #include <linux/usb/ehci_def.h>
 #include <linux/delay.h>
 #include <linux/serial_core.h>
+#include <linux/kconfig.h>
 #include <linux/kgdb.h>
 #include <linux/kthread.h>
 #include <asm/io.h>
@@ -614,12 +615,6 @@ err:
 	return -ENODEV;
 }
 
-int dbgp_external_startup(struct usb_hcd *hcd)
-{
-	return xen_dbgp_external_startup(hcd) ?: _dbgp_external_startup();
-}
-EXPORT_SYMBOL_GPL(dbgp_external_startup);
-
 static int ehci_reset_port(int port)
 {
 	u32 portsc;
@@ -979,6 +974,7 @@ struct console early_dbgp_console = {
 	.index =	-1,
 };
 
+#if IS_ENABLED(CONFIG_USB_EHCI_HCD)
 int dbgp_reset_prep(struct usb_hcd *hcd)
 {
 	int ret = xen_dbgp_reset_prep(hcd);
@@ -1007,6 +1003,13 @@ int dbgp_reset_prep(struct usb_hcd *hcd)
 }
 EXPORT_SYMBOL_GPL(dbgp_reset_prep);
 
+int dbgp_external_startup(struct usb_hcd *hcd)
+{
+	return xen_dbgp_external_startup(hcd) ?: _dbgp_external_startup();
+}
+EXPORT_SYMBOL_GPL(dbgp_external_startup);
+#endif /* USB_EHCI_HCD */
+
 #ifdef CONFIG_KGDB
 
 static char kgdbdbgp_buf[DBGP_MAX_PACKET];