usb: Remove broken optimisation in OHCI IRQ handler

The OHCI IRQ handler has an optimisation that avoids reading some
chip registers when the controller reports that the interrupt was
triggered *only* because completed requests were written into the
controller's "done list" and handed to the host.

This mechanism can't be used on some controllers.  Among others, it
fails for the SA1111 and the AMCC 440EP PowerPC processor.

This patch removes the optimisation and makes the code clearer.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>