pata_amd: fix sparse warning

Current code is essentially choosing between dividing by 1 or
dividing by two, make the conditions a little more obvious.

As a bonus, removes a sparse error:
drivers/ata/pata_amd.c:59:11: warning: symbol '__x' shadows an earlier one
drivers/ata/pata_amd.c:59:11: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>