firmware: turris-mox-rwtm: Do not complete if there are no waiters

Do not complete the "command done" completion if there are no waiters.
This can happen if a wait_for_completion() timed out or was interrupted.

Fixes: 389711b3 ("firmware: Add Turris Mox rWTM firmware driver")
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>