[imu] aspirin_v2.2: unselect baro at startup

- prevent floating CS line on the MS5611 baro connected via SPI
  so it also works if the baro is actually not read
- this requires to have the appropriate SPI_SLAVE defined in the board file
  which is the case for the lisa autopilots, but not for tiny/twog, etc

I think this is a nicer solution than what I did in #369 where PC13 was explicitly set to input with pullup in the arch file. But depending on initialization order that would break the usage of the ms5611 baro via spi.

Would be great if someone with an aspirin2.2 could test this (with and without using the baro)