usb: amd5536udc: fixed shared interrupt bug and warning oops

- fixed shared interrupt bug reported by Vadim Lobanov
 - fixed possible warning oops on driver unload when connected
 - prevent interrupt flood in PIO mode ("modprobe amd5536udc use_dma=0")
   when using gadget ether

Signed-off-by: Thomas Dahlmann <dahlmann.thomas@arcor.de>
Cc: Robert Richter <robert.richter@amd.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>