net: dsa: bcm_sf2: Use device managed helpers

Now that we have converted the drivers into a proper platform device
driver, we can use the device managed helper functions to simplify the
error paths a bit wrt. register resources and IRQs.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>