samples: subsys: usb_c: sink: Add USBC_CSM_SINK_ONLY Config to prj.conf

Add CONFIG_USBC_CSM_SINK_ONLY=y to Sink's sample prj.conf.
This selects the Sink only Connection state machine
for the Sink sample and it explicitly show the user
of the Sink sample how the connection state machines are
selected.

Signed-off-by: Sam Hurst <sbh1187@gmail.com>