ixgbe: Use pci_get_dsn()

Replace the open-coded implementation for reading the PCIe DSN with
pci_get_dsn().

The original code used a simple for-loop to read the bytes in order into
a buffer one byte at a time.

The pci_get_dsn() function returns the DSN as a u64, correctly ordering
the upper and lower 32 bit dwords. Simplify the display code by using
%016llX to display the u64 DSN.

This should have equivalent behavior on both Little and Big Endian
systems. The bus will have correctly ordered the dwords in the CPU
endian format, while pci_get_dsn() will correctly order the lower and
higher dwords into a u64.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>