rpmsg: glink: Release driver_override

Upon termination of the rpmsg_device, driver_override needs to be freed
to avoid leaking the potentially assigned string.

Fixes: 42cd402b ("rpmsg: Fix kfree() of static memory on setting driver_override")
Fixes: 39e47767 ("rpmsg: Add driver_override device attribute for rpmsg_device")
Reviewed-by: Chris Lew <quic_clew@quicinc.com>
Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230109223931.1706429-1-quic_bjorande@quicinc.com