i2c: fsi: Prevent adding adapters for ports without dts nodes

Ports should be defined in the devicetree if they are to be enabled on
the system.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Wolfram Sang <wsa@kernel.org>