Add support for AVR boards in UNIT_4RELAY::begin

[m0spak]

This change adds compatibility with AVR-based Arduino boards,
including classic boards such as UNO, Nano, and Mega.

The original implementation calls:

Wire.begin(sda, scl, freq)

However, the AVR Wire library does not support passing SDA/SCL
pins to begin(); it only supports:

Wire.begin()

This update conditionally calls Wire.begin() on AVR platforms,
while keeping the original behavior for all other boards
(ESP32, ESP8266, RP2040, etc.).

Supporting AVR boards improves portability and allows the library
to be used across a wider range of Arduino boards in the community.