ide serverworks warning fixes

gcc-4.3:

drivers/ide/pci/serverworks.c: In function 'svwks_tune_chipset':
drivers/ide/pci/serverworks.c:176: warning: overflow in implicit constant conversion
drivers/ide/pci/serverworks.c:190: warning: overflow in implicit constant conversion
drivers/ide/pci/serverworks.c:211: warning: overflow in implicit constant conversion
drivers/ide/pci/serverworks.c:212: warning: overflow in implicit constant conversion

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>