diag: rpmsg: Remove set_drvdata in close work

The device of the rpmsg handle will be freed after returning from the
rpmsg remove call. The diag close work should not try to touch any of
the internals of the rpmsg handle after returning from remove.

Change-Id: Ieaa88858a325fb55f08b3d09bcb484c9ae1e9eb4
Signed-off-by: Chris Lew <clew@codeaurora.org>