stm32: Opamp `buffer_ext` does not set `OPAMODE` to internal follower

[hvraven]

At least on the L4 series, when used as a buffer the OpAmp should be set up as a follower by writing 0b11 (internal follower) to the OPAMODE bits of the OPAMPx_CSR register. Currently they are not changed, resulting in wrong readings by sampling the second GPIO.