ocs: Check for copyin errors in the ioctl handler

If copyin() fails, the driver will blindly proceed with whatever had
been in the uninitialized DMA buffer. This is not what we want. Check
for copyin failures.

This is in preparation for annotating copyin() and related functions
with __result_use_check.

Reviewed by: ram
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43097