[media] horus3a: fix compiler warning

Shut up this compiler warning that I get during the daily build:

horus3a.c: In function 'horus3a_set_params':
horus3a.c:308:24: warning: 'rolloff' may be used uninitialized in this function [-Wmaybe-uninitialized]
     symbol_rate * (100 + rolloff), 200000) + 5;
                        ^

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>