[media] rainshadow-cec: use strlcat instead of strncat

gcc warns about an obviously incorrect use of strncat():

drivers/media/usb/rainshadow-cec/rainshadow-cec.c: In function 'rain_cec_adap_transmit':
drivers/media/usb/rainshadow-cec/rainshadow-cec.c:299:4: error: specified bound 48 equals the size of the destination [-Werror=stringop-overflow=]

It seems that strlcat was intended here, and using that makes the
code correct.

Fixes: 0f314f6c ("[media] rainshadow-cec: new RainShadow Tech HDMI CEC driver")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>