feat(i2c): It seems the i2c_read_reg function was broken and not tested

A problem was caught in the stm32_common/i2c_2.c that seem to make its way through the CI. We should add a way to catch similar bugs.

Feature request to add i2c_read_reg test, this may require an update to PHiLIP to catch this bug though...

## Related issue
https://github.com/RIOT-OS/RIOT/issues/12564