net: sparx5: Fix reading of the MAC address

There is an issue with the checking of the return value of
'of_get_mac_address', which returns 0 on success and negative value on
failure. The driver interpretated the result the opposite way. Therefore
if there was a MAC address defined in the DT, then the driver was
generating a random MAC address otherwise it would use address 0.
Fix this by checking correctly the return value of 'of_get_mac_address'

Fixes: b74ef9f9

 ("net: sparx5: Do not use mac_addr uninitialized in mchp_sparx5_probe()")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>