storm: meters from 24 up, plainer parser, documentation from the review

Storm control used meters 0 to 39, the same meters the ACL hands out,
so a storm limit and an acl meter could overwrite each other. It now
uses 24 + port * 4 + type, meters 24 to 63.

parse_storm() is no longer reentrant and uses plain locals, looks the
type up in a while loop and returns straight from the one-line cases.
storm_show() uses plain locals too.

The documentation says how IGMP decides between mcast and umcast, how
to get from a meter index to its limit, where the IFG bit of a meter
is, and what an ingress bandwidth limit on the same port does to the
storm limit.

rtl837x_storm.h

@@ -4,6 +4,8 @@
 #include <stdint.h>
 
 #define STORM_TYPES	4
+#define STORM_METER_BASE	24
+#define STORM_METER(port, type)	(STORM_METER_BASE + (((port) << 2) | (type)))
 
 void storm_set(uint8_t port, __xdata uint8_t type, __xdata uint32_t rate, __xdata uint8_t pps) __banked;
 void storm_off(uint8_t port, __xdata uint8_t type) __banked;