USB: Reset poweroff timer for all USB events

Without this, if a device is left plugged in and idle, unplugging it will
trigger an immediate shutdown.

Change-Id: I65caaa0c1473562ec5d0bb776b01d4d222d69965

firmware/usb.c

@@ -41,6 +41,7 @@
 #endif
 #include "logf.h"
 #include "screendump.h"
+#include "powermgmt.h"
 
 #ifndef BOOTLOADER
 #include "misc.h"
@@ -436,6 +437,8 @@ static void NORETURN_ATTR usb_thread(void)
     {
         queue_wait(&usb_queue, &ev);
 
+        reset_poweroff_timer(); /* Any USB event counts */
+
         switch(ev.id)
         {
         /*** Main USB thread duties ***/