irqchip/riscv-intc: Fix a typo in a pr_warn()

Anup originally re-spun his patch set to include this fix, but it was a bit too
late for my PR so I've split it out.

Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200611175302.253540-1-palmer@dabbelt.com