drivers/ide/legacy/hd.c: Array size calculation using sizeof replaced with ARRAY_SIZE

This patch replaces an array size calculation in drivers/ide/legacy/hd.c
that was done using sizeof with the ARRAY_SIZE macro.

Tested by compilation on an i386 box using "allyesconfig".
Diffed against Linus' git-tree.

Signed-off-by: Andi Drebes <lists-receive@programmierforen.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>