clk: clk_stm32f: Fix PLLSAICFGR_PLLSAIP_4 divider value

PLLSAIP divider uses 2 bits (bits 16 and 17) into RCC_PLLSAICFGR
register, available combination are :
  00: PLLSAIP = 2
  01: PLLSAIP = 4
  10: PLLSAIP = 6
  11: PLLSAIP = 8

Previously, the divider value was incorrectly set to 6.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>