net: dsa: bcm_sf2: fix unmapping registers in case of errors

In case we fail to ioremap() one of our registers, we would be leaking
existing mappings, unwind those accordingly on errors.

Fixes: 246d7f77

 ("net: dsa: add Broadcom SF2 switch driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>