net: context: Remove shadowing variable in net_context_recv()

Remove the shadowing ret variable and fix a bug that was related to
its existence - the shadowing ret variable was assigned with -ETIMEDOUT
which was supposed to be retuned by the function, but was not because
the ret variable at the function scope was left intact.

Also remove the unneded goto unlock; jump (assigning the error code is
the last operation before unlocking the mutex anyway).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>