net: dsa: mt7530: Add some return-value checks

In mt7531_cpu_port_config(), if the variable port is neither 5 nor 6,
then variable interface will be used uninitialised. Change the function
to return -EINVAL in this case.

As the return value of mt7531_cpu_port_config() is never checked
(even though it returns an int) add a check in the correct place so that
the error can be passed up the call stack. Now that we correctly handle
errors thrown in this function, also check the return value of
mt7531_mac_config() in case an error occurs here. Also add misisng
checks to mt7530_setup() and mt7531_setup(), which are another level
further up the call stack.

Fixes: c288575f ("net: dsa: mt7530: Add the support of MT7531 switch")
Addresses-Coverity: 1496993 ("Uninitialized variables")
Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>