mmc_da: fix memory leak in sddaregister()

In case we are failing to allocate mmcdata, we are returning with
a softc allocated but not attached to anything and thus leak the
memory.

Reviewed by: scottl
MFC after: 2 weeks
X-MFC: only if we also mfc other mmccam changes?
Differential Revision: https://reviews.freebsd.org/D25987