e1000e: Use pcie_capability_read_word() for reading LNKSTA

Use pcie_capability_read_word() for reading LNKSTA and remove the
custom define that matches to PCI_EXP_LNKSTA.

As only single user for cap_offset remains, replace it with a call to
pci_pcie_cap(). Instead of e1000_adapter, make local variable out of
pci_dev because both users are interested in it.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>