ixgbe: use ixgbe_read_pci_cfg_word

This patch replaces some direct uses of pci_read_config_word with the
protected ixgbe_read_pci_cfg_word function, which checks for whether the
adapter is removed when LER is enabled. We shouldn't use the
pci_read_config_word calls directly because of these checks.

This patch also cleans up an unnecessary save of a pointer to the mac
object, as our standard style is to just use the hw pointer.

CC: Arun Sharma <asharma@fb.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>