clk: bcm2835: Correct the prediv logic

If a clock has the prediv flag set, both the integer and fractional
parts must be scaled when calculating the resulting frequency.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>