rpmsg: omx: fix copy_to_user in register/unregister ioctls

The ION fd registration and deregistration ioctls has the
arguments reversed in the copy_to_user() calls. The same
has been fixed.

Also included a minor additional case for returning a NULL
ION handle back to user-space.

Change-Id: I9d45957f9b913fa1775e70618f0da82e914a41d6
Signed-off-by: Tyler Luu <tluu@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>