nfp: fix XPB register reads in debug dump

For XPB registers reads, some island IDs require special handling (e.g.
ARM island), which is already taken care of in nfp_xpb_readl(), so use
that instead of a straight CPP read.

Without this fix all "xpbm:ArmIsldXpbmMap.*" registers are reported as
0xffffffff. It has also been observed to cause a system reboot.

With this fix correct values are reported, none of which are 0xffffffff.

The values may be read using ethtool debug level 2.
 # ethtool -W <netdev> 2
 # ethtool -w <netdev> data dump.dat

Fixes: 0e6c4955

 ("nfp: dump CPP, XPB and direct ME CSRs")
Signed-off-by: Carl Heymann <carl.heymann@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>