nfp: correct desc type when header dma len is 4096

nfp: correct desc type when header dma len is 4096

When there's only one buffer to dma and its length is 4096, then
only one data descriptor is needed to carry it according to current
descriptor definition. So the descriptor type should be `simple`
instead of `gather`, the latter requires more than one descriptor,
otherwise it'll be dropped by application firmware.

Fixes: c10d12e3dce8 ("nfp: add support for NFDK data path")
Fixes: d9d950490a0a ("nfp: nfdk: implement xdp tx path for NFDK")
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Reviewed-by: Richard Donkin <richard.donkin@corigine.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20221202134646.311108-1-simon.horman@corigine.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>