usb: r8a66597-hcd: Fix up spinlock recursion in root hub polling.

The current root hub polling code exhibits a spinlock recursion on the
private controller lock. r8a66597_root_hub_control() is called from
r8a66597_timer() which grabs the lock and disables IRQs. The following
chain emerges:

  r8a66597_timer() <-- lock taken
    r8a66597_root_hub_control()
      r8a66597_check_syssts()
        usb_hcd_poll_rh_status() <-- acquires the same lock
	/* insert death here */

The entire chain requires IRQs to be disabled, so we just unlock and
relock around the call to usb_hcd_poll_rh_status() while leaving the
IRQ state unchanged.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Acked-by: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>