phy: Include NC-SI in phy setup

Add NC-SI to the usual phy handling. This makes two notable changes:
- Somewhat similar to a fixed phy, phy_connect() will create an NC-SI
phy if CONFIG_PHY_NCSI is defined.
- An early return is added to phy_read() and phy_write() to handle a
case like the NC-SI phy which does not define a bus.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>