skd: Uninitialized variable in skd_isr_completion_posted()

Someone got too agressive about removing initializations and
accidentally removed the "rc = 0;" which is required.

Fixes: c830da8c ("skd: Remove superfluous initializations from skd_isr_completion_posted()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>