Hi Wolfgang,
it finally works :-). I had to make a small modification to the x.org
source code. The file xc/programs/Xserver/hw/xfree86/i2c/xf86i2c.c
contains the initialization of the i2c bus. In function
xf86I2CBusInit, I added the two lines near the beginning:
if (b->I2CStart == NULL)
b->I2CStart = I2CStart;
which sets the start function to a default function. The problem was
that mb86290_i2c.c does not provide a start function, whereas
xf86I2CBusInit expects one under some circumstances.
Best regards,
Matthias