ixgbe: use link instead of I2C combined abstraction

Introduce ixgbe_link_operations struct with the following changes:

	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. This
also adds a new structure, ixgbe_link_info, to hold information
about the link. Initially this is just method pointers and a bus
address.

The 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.

v2 - set hw->link.ops in probe.
v3 - check ii->link_ops before setting it since we don't have it
for all devices.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>