[media] gspca - main: Take numerator into account in fps calculations

In case we ever get sub drivers which do 7.5 fps and express this as
15 / 2 fps.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>