ARM: orion5x: Fix legacy get_irqnr_and_base

Commit 5be9fc23 ("ARM: orion5x: fix legacy orion5x IRQ numbers") shifted
IRQ numbers by one but didn't update the get_irqnr_and_base macro
accordingly.  This macro is involved when CONFIG_MULTI_IRQ_HANDLER
is not defined.

[jac: 5d6bed2a

 went in to v4.2, but was backported to v3.18]

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Fixes: 5be9fc23

 ("ARM: orion5x: fix legacy orion5x IRQ numbers")
Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: Jason Cooper <jason@lakedaemon.net>