drivers: i2c: stm32: Fix routing of secondary target address

Fixes the routing of the events associated with a secondary
i2c target address being routed to the primary config. The
i2c_target_config/slave_cfg was being selected from the
driver address match but then over written by the primary.
This change fully implements the if/else of 10bit addressing
and includes a assert if the slave_cfg is NULL, and explains
why dual 10bit addresses on STM32 won't work.

Signed-off-by: Tim Woolliscroft <tim@opteran.com>