[media] gspca - main: isoc mode devices are never low speed

Quoting from the official usb 20 spec:
"5.6.4 Isochronous Transfer Bus Access Constraints

Isochronous transfers can only be used by full-speed and high-speed devices."

This means that for code paths which are isoc mode only, we don't need to
check for the device being low speed, simplifying the code.

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>