sfc: only use fixed-id if the skb asks for it

AIUI, the NETIF_F_TSO_MANGLEID flag is a signal to the stack that a
 driver may _need_ to mangle IDs in order to do TSO, and conversely
 a signal from the stack that the driver is permitted to do so.
Since we support both fixed and incrementing IPIDs, we should rely
 on the SKB_GSO_FIXEDID flag on a per-skb basis, rather than using
 the MANGLEID feature to make all TSOs fixed-id.
Includes other minor cleanups of ef100_make_tso_desc() coding style.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>