As we can never have more than ISA_NUM_IRQS (16) ISA IRQs,
replace the not very interesting hw_error() call by an
assert() which is more useful to debug condition that can
not happen.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200901104043.91383-6-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>