media: si2168: change ts bus control logic

Move the ts bus control function moved higher, enabling it
after configuring frontend and removeing ts_bus_ctrl callback.

While here, also add an error checking and re-add a comment
that were removed by commit 44587774 ("media: si2168:
Add ts bus coontrol, turn off bus on sleep").

[mchehab@s-opensource.com: I ended by applying the first version,
 instead of the right one. So, this patch contains the diff and
 the v2 changelog instead]

Fixes: 44587774 ("media: si2168: Add ts bus coontrol, turn off bus on sleep")

Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>