media: cobalt: drop an unused variable

The clock_control value is read but never actually used. Based on
a comment at the code, it has to be reset at the function.

So, drop the variable that stores its value.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>