ppc4xx: Fix problem with I2C bus >= 1 initialization

This patch fixes a problem introduced with patch eb5eb2b0
[ppc4xx: Cleanup PPC4xx I2C infrastructure]. We need to assign the I2C
base address to the "i2c" pointer inside of the controller loop.
Otherwise controller 0 is initialized multiple times instead of
initializing each I2C controller sequentially.

Tested on Katmai.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>