nfp: fix unaligned io read of capabilities word

The address of 32-bit extend capability is not qword aligned,
and may cause exception in some arch.

Fixes: 484963ce

 ("nfp: extend capability and control words")
Signed-off-by: Huanhuan Wang <huanhuan.wang@corigine.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>