mrsas: Fix callout locking in mrsas_complete_cmd()

callout_stop() requires the associated lock to be held.

This is a bit hacky, but I believe it's safe since the subsequent
mrsas_cmd_done() call will also acquire the SIM lock to stop a different
callout.

PR: 265484
Reviewed by: imp
Tested by: Jérémie Jourdin <jeremie.jourdin@advens.fr>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39559