cpci750: i2c cleanup

The following changes were made, primarily to bring the cpci750 i2c
driver in line with U-Boot's current I2C API:
- Made i2c_init() globally accessible
- Made i2c_read() and i2c_write() return an integer
- Updated i2c_init() calls to pass in CONFIG_SYS_I2C_SLAVE in the
  offhand chance someone adds slave support in the future

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>