When it comes to comparing HW switch ID strings,
use strncmp to avoid reading past the buffer.
Fixes: e86b48aa46 ("net/sfc: add HW switch ID helpers")
Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>