ide/Kconfig: fix BLK_DEV_OFFBOARD dependencies

This config option is effective only for host drivers that use
IDE_HFLAG_OFF_BOARD host flag (aec62xx, generic, hpt34x, hpt366,
pdc202xx_new, pdc202xx_old and tc86c001).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>