drivers/usbhost: Define the xHCI endpoint-allocation trace.

The xHCI driver traces endpoint allocation with XHCI_VTRACE2_EPALLOC, but
that id is in neither the enumeration nor the string table, so the driver
does not compile once verbose tracing is turned on:

  usbhost_xhci_pci.c:3285: 'XHCI_VTRACE2_EPALLOC' undeclared

It builds today only because usbhost_vtrace2() collapses to a macro that
discards its arguments unless HAVE_USBHOST_TRACE_VERBOSE is defined, which
is what stops anyone finding this until they go looking for a trace.

Add the id and the string to match, keeping the two tables in step.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Justin Hammond <justin@dynam.ac>

drivers/usbhost/usbhost_xhci_trace.c

@@ -92,6 +92,8 @@ static const struct xhci_usbhost_trace_s g_trace2[TRACE2_NSTRINGS] =
           "XHCI Hub port%d connected: %d\n"),
   TRENTRY(XHCI_VTRACE2_CTRLINOUT,
           "HXCI CTRLIN/OUT: RHPort%d req: %02x\n"),
+  TRENTRY(XHCI_VTRACE2_EPALLOC,
+          "XHCI EP%d allocated, type: %d\n"),
 };
 
 /****************************************************************************

drivers/usbhost/usbhost_xhci_trace.h

@@ -64,6 +64,7 @@ enum usbhost_trace1codes_e
   XHCI_VTRACE2_PORTSC_DISCONND,     /* XHCI RHport disconnected */
   XHCI_VTRACE2_MONWAKEUP,           /* XHCI RHPort connected wakeup */
   XHCI_VTRACE2_CTRLINOUT,           /* XHCI CTRLIN/OUT */
+  XHCI_VTRACE2_EPALLOC,             /* XHCI Endpoint allocated */
   __TRACE2_NSTRINGS                 /* Total number of enumeration values */
 };