net: net_context: correct description of recv_data_wait in net_context

The recv_data_wait field in struct net_context is described as a "Mutex"
when in fact it's a semaphore signal.

Change-Id: I3bef8d1a07ceb3da5894ae4cdc8f1fe3c61c5dbe
Signed-off-by: Michael Scott <michael.scott@linaro.org>