Disable the internal pullups after i2c initialization

You can disable the pull request after initialization of the i2c module.

The line does go a little bit above the 3.3v rail for less than 10 micro second before the first i2c packet. I doubt this would damage the si570. Even without this change, the line never goes to +5v. The 3.3v pull up resistor acts as a pull-down against the 20k internal pullup and I have measured 3.42 v max with the internal pullups on.