V4L/DVB (6893): tuner-xc2028: fix xc2028_attach function

xc2028_attach was returning an integer when disabled from the build, where it
should instead be returning NULL.  Declare xc2028_attach as type dvb_frontend *
instead of void *.

The prototype declaration must be marked as extern in the header.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>