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

MS6 and MS7 do not have the MSx_P3 field. Do the 'params.p3 == 0'
check for MS0-M5 only. See [AN619, p. 6] for details.

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>