ata: fix sparse warning in libata.h

Avoids lots of these, also is more readable.
include/linux/libata.h:1210:13: warning: potentially expensive pointer subtraction

Change the subtraction to addition on the other side of the comparison.

Thanks to Christer Weinigel for the suggestion.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Acked-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>