Fix compile error in isabus_probe with gcc 4.7

The copy of ipxe used during Xen tools build does not define
ISA_PROBE_ADDRS. As a result isa_extra_probe_addrs[] has a size of 0.
ISA_IOADDR() tries to access that empty array, which is detected by the
newer gcc (or perhaps the warning was just turned into an error)

drivers/bus/isa.c: In function 'isabus_probe':
drivers/bus/isa.c:112:18: error: array subscript is above array bounds [-Werror=array-bounds]

---
 src/drivers/bus/isa.c |    3 +++
 1 file changed, 3 insertions(+)

Index: ipxe/src/drivers/bus/isa.c
===================================================================
--- ipxe.orig/src/drivers/bus/isa.c
+++ ipxe/src/drivers/bus/isa.c
@@ -97,6 +97,9 @@ static int isabus_probe ( struct root_de
 	int ioidx;
 	int rc;
 
+	if ( ISA_EXTRA_PROBE_ADDR_COUNT == 0 )
+		return 0;
+
 	for_each_table_entry ( driver, ISA_DRIVERS ) {
 		for ( ioidx = ISA_IOIDX_MIN ( driver ) ;
 		      ioidx <= ISA_IOIDX_MAX ( driver ) ; ioidx++ ) {