staging: xillybus: Add sanity check in interrupt handler

Data arriving from the hardware is verified prior to its use. The lack of
this check has never been reported to cause a problem, but it's necessary
nevertheless.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>