mca: Switch to using taskqueue_enqueue_timeout_sbt().

Previously it was not allowed on fast taskqueues. It was fixed in
4730a8972b1f. This should make no functional change, just a bit
cleaner and efficient code.

MFC after: 1 week

(cherry picked from commit 9a128e1678d1d22a2d4855784b511c0cb370189e)