Signed bitfields are broken in `rust-bindgen`

[ProfFan]

See https://github.com/rust-lang/rust-bindgen/issues/1160

I discovered this issue in wifi_pkt_rx_ctrl_t, but same problems could be in other places as well.

Appearance of the bug: all negative values such as the RSSI are instead shown as 2's complement (-xx to 2xx)