caif_usb: use strlcpy() instead of strncpy()

gcc-8 reports

net/caif/caif_usb.c: In function 'cfusbl_device_notify':
./include/linux/string.h:245:9: warning: '__builtin_strncpy' output may
be truncated copying 15 bytes from a string of length 15
[-Wstringop-truncation]

The compiler require that the input param 'len' of strncpy() should be
greater than the length of the src string, so that '\0' is copied as
well. We can just use strlcpy() to avoid this warning.

Signed-off-by: Xiongfeng Wang <xiongfeng.wang@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>