nfp: refine the ABI of getting `sp_indiff` info

Considering that whether application firmware is indifferent to
port speed is a firmware property instead of port property, now use
a new rtsym to get the property instead of parsing per-port tlv caps.
With this change, relevant code is moved to `nfp_main` layer.

Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>