Define I2C_CLOCK_SPEED constant instead of magic number

## Problem

The clock speed  (100kHz) is hardcoded in multiple places:

-  - i2c_device_config_t
- Same value likely in bus config

## Suggested Fix

Define a named constant:

# 0 "<stdin>"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "<stdin>"

Then use it consistently throughout the code.

## File

- 