net/sfc: correct estimation of max DMA descriptors required
net/sfc: correct estimation of max DMA descriptors required

Previous too pessimistic estimation made completely impossible to send
packets with many segments (more than 100 with minimum Tx ring size).

Fixes: 8b00f426eb66 ("net/sfc: implement EF10 native Tx datapath")

Signed-off-by: Andrew Rybchenko <ac94ab2a8fe9a9a087f6e24dcbc16626b52c07e8@solarflare.com>
Reviewed-by: Andrew Lee <d14c31299facb10bc908e1762f35f6abb5a2888a@solarflare.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk