msm: mhi_dev: Adding check in open channel API

Adding check for mhi_ctx and mhi_ctx->pdev in mhi_dev_open_channel API
which is called by the clients. This prevents the device crash when
mhi_dev_open_channel is called in PCIe RC configuration or when it is
called when mhi_ctx is not created yet.

Change-Id: Ic1737fa1090c06153b0638014f014ec7c1ca2903
Signed-off-by: Sai Chaitanya Kaveti <quic_skaveti@quicinc.com>