i2c: stm32f7: do not backup read-only PECR register

The PECR register provides received packet computed PEC value. 
It makes no sense restoring its value after a reset, and anyway,
as read-only register it cannot be restored.

Fixes: ea6dd25d ("i2c: stm32f7: add PM_SLEEP suspend/resume support")
Signed-off-by: Alain Volmat <alain.volmat@st.com>
Reviewed-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>