[media] staging: atomisp: Do not call dev_warn with a NULL device

Do not call dev_warn with a NULL device, this silence the following 2
warnings:

[   14.392194] (NULL device *): Failed to find gmin variable gmin_V2P8GPIO
[   14.392257] (NULL device *): Failed to find gmin variable gmin_V1P8GPIO

We could switch to using pr_warn for dev == NULL instead, but as comments
in the source indicate, the check for these 2 special gmin variables with
a NULL device is a workaround for 2 specific evaluation boards, so
completely silencing the missing warning for these actually is a good
thing.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>