soc: qcom: ipc_router_mhi_xprt: Initialize variable at start

variable rc is not initialized in the beginning of function
ipc_router_mhi_write and ipc_router_mhi_xprt_probe. This leads
possible use without initialization issue.

Initialize variable in the beginning of both functions.

CRs-Fixed: 2209246
Change-Id: Ida0bdad2336be928d6e440d4143350bbf9f253d5
Signed-off-by: Dhoat Harpal <hdhoat@codeaurora.org>