Support ESP v2+ chip feature to divide BRP by 2

[mister0wl]

This code is a little rough, I think the best would be to calculate BRP and compare if it's too big then use the divider. But in the original codes the values of BTR0 and BTR1 is pre-calculated so I thought for first approach it fits. I could not test 20kbps so I can't be 100% confident that the values are correct.

[tzzee]

I think REG_IER is overwrite 0xFF by writeRegister I send @mister0wl a pull request