media: atomisp: Fix a buffer overflow in debug code

The "pad" variable is a user controlled string and we haven't properly
clamped it at this point so the debug code could print from beyond the
of the array.

Fixes: a49d2536 ("staging/atomisp: Add support for the Intel IPU v2")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>