habanalabs/gaudi: fix comment to reflect current code

Due to code changes in the past few years, the original comment of
how parser->user_cb_size is checked was not correct anymore.

Fix it to reflect current code and add more explanation as the code
is more complex now.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>