V4L/DVB (5976): mt2131 s5h1409: correct frontend selection logic

If a card driver is compiled into the kernel and mt2131 or s5h1409 are
compiled as modules, the kernel won't link.

A compiled in driver can't use a module, so in this case the mt2131 or s5h1409
are effectively disabled w.r.t the compiled in driver and the stub attach
function should be used.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Acked-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>