modem: pipe: simplify synchronization

The design of the pipe is overly complicated compared to the
in-tree and planned future use of the pipe module.

The pipe is currently designed to protect against multiple
threads calling any API simultaineously. This is not neccesary
as only one thread ever calls open/close/transmit/receive at
once, while the notification APIs are potentially called by a
different thread.

This commit removes the synchronization of calls to the open/
close/receive/transmit APIs. It also uses a k_event for thread
safe event and state handling instead of a k_mutex and k_condvar.

The callback is proteced by a k_sem as it modified using the
attach/release APIs, which can be called simultaneously to a
thread invoking the callback.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>