powerpc/8xx: fix cpm_cascade() dual end of interrupt

cpm_cascade() doesn't have to call eoi() as it is already called
by handle_fasteoi_irq()

And cpm_get_irq() will always return an unsigned int so the test
is useless

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>