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

The original code used a for-loop that looped over each of the 8 bytes
and copied them into a temporary buffer. pci_get_dsn() uses two calls to
pci_read_config_dword, and correctly bitwise ORs them into a u64. Thus,
we can simplify the snprintf significantly using %016llX on a u64 value.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>