ahci: fix module refcount breakage introduced by libahci split

libata depends on scsi_host_template for module reference counting and
sht's should be owned by each low level driver.  During libahci split,
the sht was left with libahci.ko leaving the actual low level drivers
not reference counted.  This made ahci and ahci_platform always
unloadable even while they're being actively used.

Fix it by defining AHCI_SHT() macro in ahci.h and defining a sht for
each low level ahci driver.

stable: only applicable to 2.6.35.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Pedro Francisco <pedrogfrancisco@gmail.com>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: stable@kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>