powerpc: MPC8544DS: revert typo in I2C offset value

I2C offset was changed by commit 00f792e0 (added multibus support)
from 0x3100 to 0x3000. This typo leads to error when reading SPD
from DDR DIMMs.

Signed-off-by: Benjamin Kamath <bkamath@spaceflight.com>
Signed-off-by: York Sun <york.sun@nxp.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>