Allow increasing baudrate

[MathiasKoch]

Currently the baudrate is limited to 230400 bps, because this is the highest rate, at which the ublox module does auto baudrate detection.

In order to allow higher rates, one would have to establish AT link with the module at 115200 bps, set the baudrate (AT+IPR), and reconfigure the UART peripheral to run at the higher baudrate.

This is however currently not possible, as the embedded-hal does not provide a way to change baudrate on the fly. See rust-embedded/embedded-hal#79