As of commit b059686a71c8, cam_periph_unmapmem() can legitimately fail,
if the copyout() operation fails. However, this failure is never
signaled to upper layers. In practice it is unlikely to occur since
cap_periph_mapmem() would most likely fail in such circumstances anyway.

However, some code reading revealed a few paths where the return value
of cam_periph_mapmem() is not checked, and this is definitely a bug.
Add error checking there and let cam_periph_unmapmem() return errors
from copyout().