logging: Avoid compile warning because of wrong type

The macro parameters need to be protected by (), otherwise it
is possible to get this kind of warning

   include/logging/log_core.h:214:6: note: expected
     ‘u32_t {aka unsigned int}’ but argument is of type ‘char *’
     void log_3(const char *str,

This can happen for example if the macro parameter is not plain
variable. Example:

  NET_DBG("Route %p nexthop %s", route,
         nexthop ? net_sprint_ipv6_addr(nexthop) : "<unknown>");

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>