This micro-optimization is not worth it.  Just always check for
existence of ->ack_intr method in ide_intr() and ide_timer_expiry().

v2:
Fix brown-paper-bag bug spotted by David D. Kilzer.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michael Schmitz <schmitz@debian.org>
Cc: "David D. Kilzer" <ddkilzer@kilzer.net>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>