ide: make CONFIG_IDE_GENERIC default to N

These days, CONFIG_IDE_GENERIC causes more confusion and
misconfiguration than it helps.  Especially so because libata is
linked after the generic driver.  Default to N.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Cc: "P.C.Chan" <pc.chan@alcatel-lucent.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>