V4L/DVB (10808): Fix typo in lnbp21.c

It was a typo in the function
name, it should be lnbh24_attach, and not lnbp24_attach.

Signed-off-by: Igor M. Liplianin <liplianin@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>