ext: stm32cube: stm32f7xx: shift I2SR field in PLLI2SCFGR register

The I2SR field should be shifted by RCC_PLLI2SCFGR_PLLI2SR_Pos when the
PLLI2SCFGR register is read or written. Without this patch the PLLI2S
configuration is not done properly (R and M params are badly set) and
the PLLI2S generates bad clock waveform.

ST Bug tracker ID: 50108

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