net/sfc/base: separate limitations on Tx DMA descriptors

net/sfc/base: separate limitations on Tx DMA descriptors

Siena has limitation on maximum byte count and 4k boundary crosssing
(which is stricter than maximum byte count).
EF10 has limitation on maximum byte count only.

Fixes: f7dc06bf35f2 ("net/sfc/base: import 5xxx/6xxx family support")
Fixes: e7cd430c864f ("net/sfc/base: import SFN7xxx family support")
Fixes: 94190e3543bf ("net/sfc/base: import SFN8xxx family support")
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>