[PATCH] Fix EDD to properly ignore signature of non-existing drives

Some BIOSes do not always set CF on error before return from int13.  The
patch adds additional check for status being zero (AH == 0).

Signed-off-by: Andrey Borzenkov <arvidjaar@mail.ru>
Cc: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>