Small patch to fix following warning with CONFIG_IDE && !CONFIG_PCI:

  CC	drivers/ide/ide.o
drivers/ide/ide.c: In function 'ide_system_bus_speed':
drivers/ide/ide.c:338: warning: unused variable 'pci_default'

I decided to save some bytes by #ifdef:ing the struct in question.
CC:ing Hanna because she did the change (and just to say hi ;-).
Signed-off-by: Mika Kukkonen <mikukkon@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>