net/nfp: fix buffer overflow of FW strings

net/nfp: fix buffer overflow of FW strings

drivers/net/nfp/nfp_net.c: In function ‘nfp_pf_pci_probe’:
drivers/net/nfp/nfp_net.c:3160: 23: error:
‘%s’ directive writing up to 99 bytes into a region of size 76
[-Werror=format-overflow=]
  sprintf(fw_name, "%s/%s.nffw", DEFAULT_FW_PATH, serial);

Note fw_buf still has to increase somewhat even after
restricting serial[], since otherwise:

drivers/net/nfp/nfp_net.c: In function ‘nfp_pf_pci_probe’:
drivers/net/nfp/nfp_net.c:3176:23:
error: ‘%s’ directive writing up to 99 bytes into a region of size 76
[-Werror=format-overflow=]
  sprintf(fw_name, "%s/%s", DEFAULT_FW_PATH, card);
                       ^~
drivers/net/nfp/nfp_net.c:3262:32:
  err = nfp_fw_upload(dev, nsp, card_desc);
                                ~~~~~~~~~
drivers/net/nfp/nfp_net.c:3176:2:
note: ‘sprintf’ output between 25 and 124 bytes into a destination of size 100
  sprintf(fw_name, "%s/%s", DEFAULT_FW_PATH, card);

Fixes: 896c265ef954 ("net/nfp: use new CPP interface")

Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>