pata_cm64x: fix boot crash on parisc

The old IDE cmd64x checks the status of the CNTRL register to see if
the ports are enabled before probing them.  pata_cmd64x doesn't do
this, which causes a HPMC on parisc when it tries to poke at the
secondary port because apparently the BAR isn't wired up (and a
non-responding piece of memory causes a HPMC).

Fix this by porting the CNTRL register port detection logic from IDE
cmd64x.  In addition, following converns from Alan Cox, add a check to
see if a mobility electronics bridge is the immediate parent and forgo
the check if it is (prevents problems on hotplug controllers).

Signed-off-by: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>