drivers/i2s_ll_stm32.c: pass proper PLLR param when setting I2S PLL

The PLLR parameter in LL_RCC_PLLI2S_ConfigDomain_I2S() API should
be selected among the following list of (already shifted) values:

  *         @arg @ref LL_RCC_PLLI2SR_DIV_2
  *         @arg @ref LL_RCC_PLLI2SR_DIV_3
  *         @arg @ref LL_RCC_PLLI2SR_DIV_4
  *         @arg @ref LL_RCC_PLLI2SR_DIV_5
  *         @arg @ref LL_RCC_PLLI2SR_DIV_6
  *         @arg @ref LL_RCC_PLLI2SR_DIV_7

This commit fixes PR #12609.

Signed-off-by: Armando Visconti <armando.visconti@st.com>