isci(4): Use controller->lock for busdma tags.

isci(4) uses deferred loading. Typically on amd64 and i386 non-PAE
the tag does not create any restrictions, but on i386 PAE-tables but
non-PAE configs callbacks might be used.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks