gen_isr_tables: Fix pylint warning by using isinstance()

Fix this warning, as a preparation for a CI check:

    arch/common/gen_isr_tables.py:167:11: C0123: Using type() instead of
    isinstance() for a typecheck. (unidiomatic-typecheck)

isinstance() has the advantage that it also handles inheritance, though
it doesn't really matter here. It's more common at least.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>