[media] RedRat3: Delete an unnecessary variable initialisation in redrat3_get_firmware_rev()

The local variable "rc" will be set to an appropriate value a bit later.
Thus omit the explicit initialisation at the beginning.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>