isdn/hisax: fix compiler warning on hisax_pci_tbl

Annotate hisax_pci_tbl as '__used' to fix following warning:

  CC      drivers/isdn/hisax/config.o
drivers/isdn/hisax/config.c:1920: warning: ‘hisax_pci_tbl’ defined but not used

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>