[media] em28xx: don't sleep on disconnect

The DVB framework will try to power-down an adapter that no-one is using
any more, but this assumes that the adapter is still connected to the
machine. That's not always true for a USB adapter, so disable the sleep
operations when the adapter has been physically unplugged.

This prevents I2C write failures with error -19 from appearing
occasionally in the dmesg log.

Signed-off-by: Chris Rankin <rankincj@yahoo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>