ixgbe: fix semaphore lock for I2C read/writes on 82598

ixgbe_read/write_i2c_phy_82598() does not hold the SWFW_SYNC
semaphore for the entire function. Instead the lock is held only
during the phy.ops.read/write_reg operations. As result when the
function is being called simultaneously the I2C read/writes can
be corrupted.

The following patch introduces the SWFW_SYNC semaphore for the
entire ixgbe_read/write_i2c_phy_82598() function. To accomplish
this I had to create 2 separate functions:

ixgbe_read_phy_reg_mdi()
ixgbe_write_phy_reg_mdi()

Those functions are identical to ixgbe_read/write_phy_reg_generic()
sans the locking, and can be used in ixgbe_read/write_i2c_phy_82598()
with the SWFW_SYNC semaphore being held.

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>