xen/usb: Constify struct hc_driver

xen/usb: Constify struct hc_driver

'struct hc_driver' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  52065	  23176	    256	  75497	  126e9	drivers/usb/host/xen-hcd.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  52897	  22344	    256	  75497	  126e9	drivers/usb/host/xen-hcd.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/63241c9e857646d895ce615b998d41ee4829f9e3.1761475831.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>