altos: get MPU6000 I2C link working reliably

altos: get MPU6000 I2C link working reliably

This slows the i2c bus to 100kHz (yuck), sets the rise time to spec
(it was way off) and adds more delays during bus setup. I've run this
for hours now without trouble. Will try to adjust things back to fast
mode and see if I can make that work as 100kHz isn't fast enough to
reliably get data at 100 samples/sec.

Signed-off-by: Keith Packard <keithp@keithp.com>