xtensa: iss/network: make functions static

Make 2 functions static to prevent build warnings:

arch/xtensa/platforms/iss/network.c:204:16: warning: no previous prototype for 'tuntap_protocol' [-Wmissing-prototypes]
  204 | unsigned short tuntap_protocol(struct sk_buff *skb)
arch/xtensa/platforms/iss/network.c:444:6: warning: no previous prototype for 'iss_net_user_timer_expire' [-Wmissing-prototypes]
  444 | void iss_net_user_timer_expire(struct timer_list *unused)

Fixes: 7282bee7 ("xtensa: Architecture support for Tensilica Xtensa Part 8")
Fixes: d8479a21

 ("xtensa: Convert timers to use timer_setup()")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230920052139.10570-14-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>