ice: fix PCI device serial number to be lowercase values

Commit ceb2f007 ("ice: Use pci_get_dsn()") changed the code to
use a new function to get the Device Serial Number. It also changed
the case of the filename for loading a package on a specific NIC
from lowercase to uppercase. Change the filename back to
lowercase since that is what we specified.

Fixes: ceb2f007 ("ice: Use pci_get_dsn()")
Signed-off-by: Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>