[media] dvb-usb: increase MAX_NO_OF_FE_PER_ADAP from 2 to 3

The following patch adds support for a third frontend to exist on a single
DVB adapter, in the mxl111sf driver. This patch allows that to be possible.

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