Net: DSA: Fix checking for get_phy_flags function

The check for the presence or not of the optional switch function
get_phy_flags() called the function, rather than checked to see if it
is a NULL pointer. This causes a derefernce of a NULL pointer on all
switch chips except the sf2, the only switch to implement this call.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Fixes: 6819563e ("net: dsa: allow switch drivers to specify phy_device::dev_flags")
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>