msm: ipc: Post a packet to a local port using a common routine

A packet is posted to a local port from multiple locations within the
IPC router code. This causes the same code to be repeated in multiple
locations.

Add a common routine to perform that operation, reduce the code
redundancy and avoid any potential bug due to incomplete code repetition.

Change-Id: I4ed43feb3ea8bb88479b89814846d5ea20bb3665
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>