ide/Kconfig: BLK_DEV_4DRIVES should be a tristate

The 4drives should be a tristate, the following breakage occurs otherwise:

ERROR: "probe_4drives" [drivers/ide/ide-core.ko] undefined!

The following (compile tested) patch fixes it for me.

Signed-off-by: Frederik Deweerdt <frederik.deweerdt@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>