[TG3]: Add tagged status support.

When supported, use the TAGGED interrupt processing support
the chip provides.  In this mode, instead of a "on/off" binary
semaphore, an incrementing tag scheme is used to ACK interrupts.

All MSI supporting chips support TAGGED mode, so the tg3_msi()
interrupt handler uses it unconditionally.  This invariant is
verified when MSI support is tested.

Since we can invoke tg3_poll() multiple times per interrupt under
high packet load, we fetch a new copy of the tag value in the
status block right before we actually do the work.

Also, because the tagged status tells the chip exactly which
work we have processed, we can make two optimizations:

1) tg3_restart_ints() need not check tg3_has_work()
2) the tg3_timer() need not poke the chip 10 times per
   second to keep from losing interrupt events

Based upon valuable feedback from Michael Chan <mchan@broadcom.com>

Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tg3.h

@@ -2023,6 +2023,7 @@ struct tg3 {
 
 	struct tg3_hw_status		*hw_status;
 	dma_addr_t			status_mapping;
+	u32				last_tag;
 
 	u32				msg_enable;
 
@@ -2068,6 +2069,7 @@ struct tg3 {
 
 	u32				rx_offset;
 	u32				tg3_flags;
+#define TG3_FLAG_TAGGED_STATUS		0x00000001
 #define TG3_FLAG_TXD_MBOX_HWBUG		0x00000002
 #define TG3_FLAG_RX_CHECKSUMS		0x00000004
 #define TG3_FLAG_USE_LINKCHG_REG	0x00000008