Overwrite an already set proto if a new one is passed to
_ucidef_set_interface() similar to what is done for the interface.

It is required when using ""ucidef_set_interface_wan 'ptm0' 'pppoe'"
after some initial wan interface configuration is already done by
ucidef_add_switch.

The "json_is_a protocol string" guard is meant to not reset an earlier
set interface proto in case something like
"ucidef_set_interface_lan 'eth0'" is used afterwards.

Signed-off-by: Mathias Kresin <dev@kresin.me>