In arch_irq_connect_dynamic the 'level' variable is only used on
platforms that define CONFIG_RISCV_HAS_PLIC. For the other platforms
we'll get a warning about an unused variable. Remove the need for
'level' and just call irq_get_level() where its needed to address the
issue.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>