iio: adc: rcar-gyroadc: Derive interface clock speed from fck clock

The "if" interface clock speed is actually derived from the "fck"
block clock, as in the hardware they are the same clock. Drop the
incorrect second "if" clock and get the clock speed from "fck".

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: linux-renesas-soc@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>