media: i2c: ds90ub960: Fix UB9702 refclk register access

UB9702 has the refclk freq register at a different offset than UB960,
but the code uses the UB960's offset for both chips. Fix this.

The refclk freq is only used for a debug print, so there's no functional
change here.

Cc: stable@vger.kernel.org
Fixes: afe267f2 ("media: i2c: add DS90UB960 driver")
Reviewed-by: Jai Luthra <jai.luthra@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>