iio: adc: rockchip_saradc: remove unused variable in probe

The rate variable in the probe function of the saradc is a remnant
of a previous patch iteration. It is unused and thus produces a
compile time warning. Therefore remove it.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>