media: usb: don't initialize vars if not needed

Some local variables will be set to an appropriate value before usage.
Thus omit explicit initialisations at the beginning of these functions.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>