nfp: use correct macro for LengthSelect in BAR config

The 1st and 2nd expansion BAR configuration registers are configured,
when the driver starts up, in variables 'barcfg_msix_general' and
'barcfg_msix_xpb', respectively. The 'LengthSelect' field is ORed in
from bit 0, which is incorrect. The 'LengthSelect' field should
start from bit 27.

This has largely gone un-noticed because
NFP_PCIE_BAR_PCIE2CPP_LengthSelect_32BIT happens to be 0.

Fixes: 4cb584e0 ("nfp: add CPP access core")
Cc: stable@vger.kernel.org # 4.11+
Signed-off-by: Daniel Basilio <daniel.basilio@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>