media: cec: extron-da-hd-4k-plus: Fix Wformat-truncation

Fix gcc8 warning:

drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c:1014:44: warning: 'DCEC' directive output may be truncated writing 4 bytes into a region of size between 0 and 53 [-Wformat-truncation=]

Resizing the 'buf' and 'cmd' arrays fixed the warning.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>