mcp23xxx: fix invalid value; add link to datasheet

See page 12, Table 3-1, and page 17 table 3-5 (bank select is zero on
reset).

experimental/devices/mcp23xxx/doc.go

@@ -3,4 +3,8 @@
 // that can be found in the LICENSE file.
 
 // Package mcp23xxx provides driver for the MCP23 family of IO extenders
+//
+// Datasheet
+//
+// https://ww1.microchip.com/downloads/en/DeviceDoc/20001952C.pdf
 package mcp23xxx

experimental/devices/mcp23xxx/mcp23xxx.go

@@ -152,7 +152,7 @@ func mcp23x178ports(devicename string, ra registerAccess) []port {
 
 		// interrupt handling registers
 		gpinten:          ra.define(0x05),
-		intcon:           ra.define(0x0B),
+		intcon:           ra.define(0x09),
 		intf:             ra.define(0x0F),
 		intcap:           ra.define(0x11),
 		supportInterrupt: true,