net: systemport: Fix sparse warnings in bcm_sysport_insert_tsb()

skb->protocol is a __be16 which we would be calling htons() against,
while this is not wrong per-se as it correctly results in swapping the
value on LE hosts, this still upsets sparse. Adopt a similar pattern to
what other drivers do and just assign ip_ver to skb->protocol, and then
use htons() against the different constants such that the compiler can
resolve the values at build time.

Fixes: 80105bef ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>