V4L/DVB (11343): au0828: make i2c clock speed per-board configurable

Setup the i2c clock speed to be definable on a per-board basis.  This allows
us to explicitly set the clock speed to 30 KHz on the 950q, and also gets rid
of code which sets it on a basis of what chip the i2c master is talking to
at any given time (which could have caused issues because i2c slaves should
never receive commands at a clock higher than their supported clock speed).

Signed-off-by: Devin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>