Fix -Wformat warning

Fixes:
 pi_boards.c:190:56: warning: format '%x' expects argument of type
 'unsigned int', but argument 4 has type 'long unsigned int' [-Wformat=]
   190 |         fprintf(stderr, "Unexpected cookie at %04x..%04x:\n%08x %08x %08x\n",
       |                                                     ~~~^
       |                                                        |
       |                                                        unsigned int
       |                                                     %04lx

Signed-off-by: Damian Wrobel <dwrobel@ertelnet.rybnik.pl>