nfp: Use pci_get_dsn()

Use the newly added pci_get_dsn() function for obtaining the 64-bit
Device Serial Number in the nfp6000_read_serial and
nfp_6000_get_interface functions.

pci_get_dsn() reports the Device Serial number as a u64 value created by
combining two pci_read_config_dword functions. The lower 16 bits
represent the device interface value, and the next 48 bits represent the
serial value. Use put_unaligned_be32 and put_unaligned_be16 to convert
the serial value portion into a Big Endian formatted serial u8 array.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>