V4L/DVB (6357): pvrusb2: Improve encoder chip health tracking

This is a minor change to help with tracking the viability of the
encoder chip within the PVR USB2 device.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>