media: rtl28xxu: add missing sleep before probing slave demod

Slave demod needs some time to wake up otherwise it may not respond to the
following probe commands. This problem manifested randomly on my Astrometa
DVB-T2 dongle.

Signed-off-by: Mario Hros <git@reversity.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>