net: microchip: Demote non-compliant kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/net/ethernet/microchip/encx24j600.c:3: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Cc: Jon Ringle <jringle@gridpoint.com>
Signed-off-by: Yang Shen <shenyang39@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>