ixgbe: add write flush in ixgbe_clock_out_i2c_byte()

I2C access is timing critical. Always do a write flush after writing
to the I2CCTL register.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>