clk: si5351: remove variant from platform_data

Commit 9807362b
  "clk: si5351: declare all device IDs for module loading"
removed the common i2c_device_id and introduced new ones for each variant
of the clock generator. Instead of exploiting that information in the driver,
it still depends on platform_data passing the chips .variant.

This removes the now redundant .variant from the platform_data and puts it in
i2c_device_id's .driver_data instead.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>