lib: irqchip/plic: fix typo in plic_warm_irqchip_init

The second invocation of plic_context_init() incorrectly calls the
function with m_cntx_id instead of s_cntx_id. This breaks systems which
only have 1 external interrupt per hart.

Fixes: 8c362e7d

 ("lib: irqchip/plic: Factor out a context init function")
Signed-off-by: Jan Remes <jan.remes@codasip.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Samuel Holland <samuel@sholland.org>