Make source buffers const for char be write functions.
This allows using buffers returned by fifo as buf parameter and source buffer
should not be changed by write functions anyway.

Signed-off-by: Arwed Meyer <arwed.meyer@gmx.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220911181840.8933-3-arwed.meyer@gmx.de>