ixgbe: cleanup handling of I2C interface to PHY

The I2C interface was not being correctly locked down per port.  As such
this can lead to race conditions that can cause issues.  This patch cleans
up the handling to make certain we are not experiencing racy I2C access.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>