USB: host: ohci-at91: fix a crash in ohci_hcd_at91_overcurrent_irq

The interrupt handler, ohci_hcd_at91_overcurrent_irq may be called right
after registration. At that time, pdev->dev.platform_data is not yet set,
leading to a NULL pointer dereference.

Fixes: e4df9227 (USB: host: ohci-at91: merge loops in ohci_hcd_at91_drv_probe)
Reported-by: Peter Rosin <peda@axentia.se>
Tested-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: stable@vger.kernel.org # 4.3+
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>