eth: 3c515: replace cleanup_module with __exit

eth: 3c515: replace cleanup_module with __exit

update old legacy cleanup_module from the file with __exit
module as per kernel code practices and restore the #ifdef
MODULE condition to allow successful compilation as a built
-in driver.

The file had an old cleanup_module still in use which could
be updated with __exit module function although its init_module
is indeed newer however the cleanup_module was still
using the older version of exit.

To set proper exit module function replace cleanup_module
with __exit corkscrew_exit_module to align it to the
kernel code consistency.

Signed-off-by: Shi Hao <i.shihao.999@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251018052541.124365-1-i.shihao.999@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>