Use PCI_DEVFN() and PCI_FUNC_MAX where appropriate.
This patch make it clear that func = 0.

test:
The following object files with/without this patch are stripped and compared.
They remains same.
  arm-softmmu/versatile_pci.o
  libhw32/ppce500_pci.o
  libhw32/unin_pci.o
  libhw64/ppce500_pci.o
  libhw64/unin_pci.o
  mips-softmmu/gt64xxx.o
  mips64-softmmu/gt64xxx.o
  mips64el-softmmu/gt64xxx.o
  mipsel-softmmu/gt64xxx.o

Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Yu Liu <yu.liu@freescale.com>
Cc: Paul Brook <paul@codesourcery.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>