spcom: spcom_get_next_request_size() unlock while waiting for rx data

spcom server app might open the channel before the SP client app.
The asynchronous CONNECTED callback might be called while the server
is waiting for data.
Avoid locking the channel while waiting for data to avoid dead lock
while handling the CONNECTED callabck.

Change-Id: I7c576f240b60720a6f1461b3e48f4422c110e7c4
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>