drivers: i2c: stm32: fix build error after timeout patch

Follow-up to PR #88631: fix missing variable declarations introduced in
the STM32 I2C LL driver after adding timeouts to blocking loops.

The missing declarations caused a build failure when interrupts were
disabled (CONFIG_I2C_STM32_INTERRUPT=n).

Fixes a regression introduced in #88631.

Signed-off-by: Jean Nanchen <jean.nanchen@gmail.com>