net: phy: phy driver features are mandatory

Since phy driver features became a link_mode bitmap, phy drivers that
don't have a list of features configured will cause the kernel to crash
when probed.

Prevent the phy driver from registering if the features field is missing.

Fixes: 719655a1 ("net: phy: Replace phy driver features u32 with link_mode bitmap")
Reported-by: Scott Wood <oss@buserror.net>
Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>