net/ncsi: Introduce NCSI_RESERVED_CHANNEL

This defines NCSI_RESERVED_CHANNEL as the reserved NCSI channel
ID (0x1f). No logical changes introduced.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>