staging: unisys: visorbus: relocate error-check from isr to registration

It just makes more sense to do the NULL-pointer check when the function is
called to enable interrupts, rather than on *every* interrupt.

Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>