char: xillybus: Fix trivial bug with mutex

@unit_mutex protects @unit from being freed, so obviously it should be
released after @unit is used, and not before.

This is a follow-up to commit 282a4b71

 ("char: xillybus: Prevent
use-after-free due to race condition") which ensures, among others, the
protection of @private_data after @unit_mutex has been released.

Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20221117071825.3942-1-eli.billauer@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>