i2c-davinci: Initialize cmd_complete sooner

If an interrupt happens before an I2c master read/write,
complete is called on uninitialized structure.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>