net/nfp: remove pessimistic limit

net/nfp: remove pessimistic limit

Multiple writes cause intermediate pointer values that do not
end on complete TX descriptors.

The QCP peripheral on the NFP provides a number of access
modes. In some access modes, the maximum amount to add must
be restricted to a 6bit value. The particular access mode
used by _nfp_qcp_ptr_add() has no such restrictions, so the
"NFP_QCP_MAX_ADD" test is unnecessary.

Note that trying to add more that the configured ring size
in a single add will cause a QCP overflow, caught and handled
by the QCP peripheral.

Signed-off-by: Jin Liu <jin.liu@corigine.com>
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Chaoyong He <chaoyong.he@corigine.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>