[media] drxd: fix divide error

Fix division by zero in drxd triggered by running "femon" before any DVB
tuning has been done (by "scandvb" or anything else).

Signed-off-by: Edward Sheldrake <ejsheldrake@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>