staging: xillybus: Non-interruptible mutex_lock() on release method

Responding to an interrupt while handling the "release" method can't end
well anyhow. In practical cases, this can force the user to wait up to
one second while flushing remaining data is attempted (a timeout on the flush
mechanism limits the time for flushing).

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>