clk: si5351: fix .round_rate for multisynth 6-7

The divider calculation for multisynth 6 and 7 differs from the
calculation for multisynth 0-5.

For MS6 and MS7, set MSx_P1 directly, MSx_P1=divide value
[AN619, p. 6].

Referenced document:
[AN619] Manually Generating an Si5351 Register Map, Rev. 0.4

Signed-off-by: Sergej Sawazki <ce3a@gmx.de>
Signed-off-by: Michael Turquette <mturquette@linaro.org>