To help with debugging i2c-over-aux issues, add a module parameter than
can be used to tweak the assumed i2c bus speed, and thus the maximum
number of retries we will do for each aux message.

Cc: Simon Farnsworth <simon.farnsworth@onelan.com>
Cc: moosotc@gmail.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Simon Farnsworth <simon.farnsworth@onelan.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>