i2c: stm32: add DMA callback stubs to avoid nullptr calls in ISR context

Add stub functions for the I2C DMA callbacks, which are invoked
during or upon completion of DMA-based I2C transactions. Without
these, NULL pointer calls occur on DMA transfer complete or error
events, leading to faults within ISR context.

Signed-off-by: Simon Gilbert <srdgilbert@gmail.com>