net/smc: define a reserved CHID range for virtual ISM devices

According to virtual ISM support feature defined by SMCv2.1, CHIDs in
the range 0xFF00 to 0xFFFF are reserved for use by virtual ISM devices.

And two helpers are introduced to distinguish virtual ISM devices from
the existing platform firmware ISM devices.

Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Reviewed-and-tested-by: Wenjia Zhang <wenjia@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>