i2c: octeon: Add flush writeq helper function

Add helper function that reads back a value after writing to
make sure the write is finished and use it in octeon_i2c_write_int().

Signed-off-by: Peter Swain <pswain@cavium.com>
Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>