net: stmmac: use sysfs_streq() instead of strncmp()

Replace the open-code with sysfs_streq().

Signed-off-by: Xu Panda <xu.panda@zte.com.cn>
Signed-off-by: Yang Yang <yang.yang29@zte.com>
Signed-off-by: David S. Miller <davem@davemloft.net>