media: mxl692: remove impossible condition

Fix the following coverity warning:

This greater-than-or-equal-to-zero comparison of an unsigned value is
always true. "opcode >= 0".

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>