media: mantis: fix two smatch errors

Drop two dprintk's that relied on a non-NULL mantis pointer
when it was in fact a NULL pointer.

Fix those warnings:

drivers/media/pci/mantis/mantis_cards.c:73 mantis_irq_handler() error: we previously assumed 'mantis' could be null (see line 72)
drivers/media/pci/mantis/hopper_cards.c:64 hopper_irq_handler() error: we previously assumed 'mantis' could be null (see line 63)

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>