net: qrtr: mhi_dev: Handle mhi register error case

Handle -EEXIST error case while register state callback
with mhi device. If channel is open during registration
no callback will issue instead mhi stack return -EEXIST
to notify the client. Further state change notification
will be informed via registered callback.

Change-Id: Icf60de35d23393a0808ef90058c1108e950d02b9
Signed-off-by: Arun Prakash <app@codeaurora.org>