media: em28xx: Increase max em28xx boards to max dvb adapters

Maximum 4 em28xx boards is too low, this can be maxed out by two devices.
This allows all the dvb adapters in the system to be em28xx if so desired.

Signed-off-by: Brad Love <brad@nextdimension.cc>
Reviewed-by: Michael Ira Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>