net: fec: replace snprintf() with strlcpy() in fec_ptp_init()

No need to use snprintf() on a constant string, nor using magic
constant in the fixed code was a good idea.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>