USB: refactor the powermac-specific ASIC clock code

This patch (as1383) takes the powermac-specific code from the PCI HCD
glue layer and encapsulates it in its own subroutine.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>