mpc83xx: Fix empty i2c reads/writes in fsl_i2c.c

Fix empty i2c reads/writes, i2c_write(0x50, 0x00, 0, NULL, 0)
which is used to se if an slave will ACK after receiving its address.

Correct i2c probing to use this method as the old method could upset
a slave as it wrote a data byte to it.

Add a small delay in i2c_init() to let the controller
shutdown any ongoing I2C activity.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>