powerpc/4xx: Cleanup Warp for i2c driver changes.

This patch removes the i2c code which is now obsolete due to the new
ibm iic driver walking the device tree for child nodes.

There are two other small cleanups that came indirectly from the ad7414
code review. Make sure Tlow is correct and handle the case where
i2c_smbus_read_word_data fails.

Signed-off-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>