soc: qcom: qmi_rmnet: Check for allocation failures

Fix a potential null de-reference in case the rmnet powersave
workqueue allocation fails.

Change-Id: Ie5d4fc0e50793cb4e508c8fc4b2e0fbafaf82c4b
Acked-by: Ning Cai <ncai@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>