CRAS: a2dp_io: Log mtu size at startup.

Add the mtu size to the print when opening an A2DP iodev.

BUG=chromium:244615
TEST=none

Change-Id: Id5af344cc02f9d2bfeb19a10c1c3c9c29fff78dd
Signed-off-by: Dylan Reid <dgreid@chromium.org>