Force enable and reset to be low when deiniting chip

[ircmaxell]

Currently, when deiniting the WiFi chip via WiFi.end(), the enable pin is left floating.

As per the data sheet section 8.4 (Power up/down sequence), both the enable and reset pins must be asserted high or low, and never left floating:

CHIP_EN must not rise before VDDIO. CHIP_EN must be driven high or low, not left floating

RESETN must be driven high or low, not left floating.

This PR addresses this by ensuring both pins are output (enable was INPUT_PULLUP), and force asserting both pins low on deinit. This appears safe, as the enable and reset pins do not appear to be used by any other piece of code in the WiFi101 library.

[CLAassistant]

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[ircmaxell]

When I click the link, it does not show me an eligible version of the CLA to sign for. Just a blank list.