dfu: cmd: trigger watchdog before calling usb_gadget_handle_interrupts

trigger watchdog before calling usb_gadget_handle_interrupts()
This prevents board resets when calling dfu command on boards
which have a watchdog.

Signed-off-by: Heiko Schocher <hs@denx.de>
[ Reedition by Lukasz Majewski <l.majewski@samsung.com> to apply to
v2014.04 release ]