soc: qcom: glink_pkt: Fix priv data for rpmsg cb

Use the device driver private helper to get the glink packet device
from the rpmsg callback instead of using the private variable from
the callback.

Change-Id: I27398995c8372021c631f082639a35c338ca134c
Signed-off-by: Chris Lew <clew@codeaurora.org>