This symbol is not used outside of net.c, so marks it static.

Fix the following sparse warning:

drivers/net/mhi/net.c:385:23: warning: symbol 'mhi_wwan_ops' was not
declared. Should it be static?
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>