[media] media: solo6x10: fix lockup by avoiding delayed register write

This fixes a lockup at device probing which happens on some solo6010
hardware samples. This is a regression introduced by commit e1ceb25a
("[media] SOLO6x10: remove unneeded register locking and barriers")

The observed lockup happens in solo_set_motion_threshold() called from
solo_motion_config().

This extra "flushing" is not fundamentally needed for every write, but
apparently the code in driver assumes such behaviour at last in some
places.

Actual fix was proposed by Hans Verkuil.

Fixes: e1ceb25a ("[media] SOLO6x10: remove unneeded register locking and barriers")

Cc: stable@vger.kernel.org  # 4.3 and up
Signed-off-by: Andrey Utkin <andrey.utkin@corp.bluecherry.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>