ixgbe/base: abstract out link read/write

ixgbe/base: abstract out link read/write

It's more valuable to abstract the link read/write interface. As such,
change the following method names, and add them to a new link info
structure:
  read_i2c_combined               => read_link
  read_i2c_combined_unlocked      => read_link_unlocked
  write_i2c_combined              => write_link
  write_i2c_combined_unlocked     => write_link_unlocked

This will allow X550EM_a to override these methods for MDIO access
while X550EM_x provides methods to use I2C combined access.
Initially the structure is just method pointers and a bus
address.

Two functions involved in combined I2C accesses were moved from
ixgbe_phy.c to ixgbe_x550.c. The underlying functions that carry
out the combined I2C accesses were left in ixgbe_phy.c because
they share some functions with other I2C methods.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>