sfc/siena: Remove duplicate check on segments

Siena only supports software TSO. This means more code can be deleted,
as pointed out by the Smatch static checker warning:
	drivers/net/ethernet/sfc/siena/tx.c:184 __efx_siena_enqueue_skb()
	warn: duplicate check 'segments' (previous on line 158)

Fixes: 956f2d86 ("sfc/siena: Remove build references to missing functionality")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/kernel-janitors/YoH5tJMnwuGTrn1Z@kili/


Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Link: https://lore.kernel.org/r/165294463549.23865.4557617334650441347.stgit@palantir17.mph.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>