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
f1c6872e.

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>