skd: Fix a function name in a comment

There is no function skd_completion_posted_isr() in the skd driver
but there is a function called skd_isr_completion_posted(). Fix
the function name in the comment.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>