Use STM32_CORE_VERSION to detect stm32duino platform

[marshfolx]

When using stm32duino with Platformio, STM32 macro is not defined. It's more reliable to use STM32_CORE_VERSION instead.

// #elif defined(STM32)
#elif defined(STM32_CORE_VERSION)

#define I2C_BUFFER_LENGTH_RX BUFFER_LENGTH // BUFFER_LENGTH is defined in Wire.h for STM32
#define I2C_BUFFER_LENGTH_TX BUFFER_LENGTH

[nseidle]

Thanks for your PR! This has been merged.