USB: at91_udc: correct hanging while disconnecting usb cable

Correct hanging while disconnecting the USB device cable.  Prevent a race
between vbus and UDP interrupts.  This bug was tracked on at91sam9260ek
boards.

A usb resume interrupt was firing after the vbus interrupt : the IP was
then already stoped and not able to deal with it (no more clock).  A simple
interrupt disabling is ok as the "end of bus reset" irq is non maskable and
ok to resume the USB device IP.

Signed-off-by: Nicolas Ferre <nicolas.ferre@rfo.atmel.com>
Acked-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>