ice: Use pci_get_dsn()

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

The pci_get_dsn() function will perform two pci_read_config_dword calls
to read the lower and upper config dwords. It bitwise ORs them into
a u64 value. Instead of using put_unaligned_le32 to convert the value to
LE32 format, just use the %016llX printf specifier. This will print the
u64 correct, putting the most significant byte of the value first. Since
pci_get_dsn() correctly orders the two dwords into a u64, this should
produce equivalent results in less code.

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>