cmake: s/CONFIG_I2C_STM32_V1x/CONFIG_I2C_STM32_V1/

Fix typo in build scripts for STM's I2C driver. The typo was
introduced in the cmake migration.

Discovered by Dmitry:
https://lists.zephyrproject.org/pipermail/zephyr-devel/2017-November/008383.html



Signed-off-by: Sebastian Boe <sebastian.boe@nordicsemi.no>