libata: pata_pdc2027x PLL detection minor cleanup

Minor cleanup to remove the unneeded rmb()s per Jeff's advice. Also removed the
pll_clock < 0 check since pll_clock now guaranteed to be >= 0 after Mikael's patch.

Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>