lib: string: Fix strlcpy return value

strlcpy should always return the number of bytes copied. We were
accidentally missing the nul-terminator. We also always used to return a
non-zero value, even if we did not actually copy anything.

Fixes: 23cd1385 ("Integrate USB gadget layer and USB CDC driver layer")

Signed-off-by: Sean Anderson <seanga2@gmail.com>