wasapi: Fix double call to Start when resetting the element

When either the source or sink goes from PLAYING -> NULL -> PLAYING,
we call _reset() which sets client_needs_restart, and then we call
prepare() which calls IAudioClient_Start(), so we don't need to call
it again in src_read() or sink_write(). Unlike when we're just going
6 jobs for fix-wasapi-double-start in 27 minutes and 11 seconds