pata_pdc2027x: PLL detection fixes

pata_pdc2027x: PLL detection fixes

Previously I reported that the pata_pdc2027x PLL detection changes
in kernel 2.6.22 broke the driver on my PowerMac:

>pata_pdc2027x: Invalid PLL input clock 1691742kHz, give up!

This is followed by a number of errors and speed reduction
steps on the affected ports.

There are two bugs in pata_pdc2027x's PLL detection code:

1. The PLL counter's start value is read before the chip is
   put in "test mode". Outside of test mode the counter is
   halted, and on the PowerMac the counter is zero because
   the chip hasn't been initialised by its BIOS.

   The fix is to move the read of the start value to after
   test mode is started, but before the mdelay() in test mode.
   This also improves the precision of the PLL detection.

2. The code to compute the number of PLL decrements during the
   mdelay() in test mode fails to consider that the PLL counter
   only is 30 bits wide. If there is a wraparound, it will compute
   an incorrect and much too large value. On the PowerMac, the
   start count is zero, the end count is a large 30-bit value, so
   wraparound occurs and an out of bounds PLL clock is detected.

   The fix is to mask the (start - end) computation to 30 bits.

While debugging this I also noticed that pdc_read_counter()
reads the two halves of the 30-bit PLL counter as 16-bit values,
and then combines them as if the halves only are 15 bits wide.
To avoid confusion, the halves should be read as 15-bit values.

This patch implements all three changes. It fixes the PLL detection
failure on my PowerMac, and doesn't cause any regressions on an x86
with an identical card.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Jeff Garzik <jeff@garzik.org>