staging: dwc2: use irq_return_t for interrupt handlers

The top-level hcd interrupt handlers already used irq_return_t, but the
functions to which it delegates the actual work and the common irq
handler returned plain ints. In addition, they used the IRQ_RETVAL in
the wrong way (but because of the values of the various constants, this
didn't result in wrong behaviour).

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
[matthijs@stdin.nl: Split patch from bigger patch and added commit message]
Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>