[imu] aspirin_2_spi: wait 1.5s before configuring mag

Seems that sometimes the mag is not correctly set to continuous measurement mode if it is configured immediately.
Delaying the configuration until the first single measurement was done seems to do the trick.

Delay configurable via ASPIRIN_2_MAG_STARTUP_DELAY

This is also nicer than using a usleep, since that would block everything else for that time.