media: pci: mgb4: fix potential spectre vulnerability

Fix smatch warnings:

drivers/media/pci/mgb4/mgb4_sysfs_out.c:118 video_source_store() warn: potential spectre issue 'mgbdev->vin' [r] (local cap)
drivers/media/pci/mgb4/mgb4_sysfs_out.c:122 video_source_store() warn: possible spectre second half.  'loopin_new'

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>