Fix memory and references leak due to unfreed path.

Coverity CID:	1109817
Unconditionally acquire periph reference on CCB allocation failure.

cam_periph_acquire() can return error if periph already invalidated, but
that may be unacceptable and cause deadlock if the invalidated periph can't
be destroyed without "executing" the scheduled request.

Coverity CID:	1109822
MFC after:	2 months