Widen the stored io_apic_id to 8 bits.

It seems that the newer Intel chipset did that, and Linux reads 8
bits. The only detail is that all seen datasheets, even under NDA,
claim that io apic id is 4 bits.

Submitted by:	jeff
Reviewed by:	jhb
Tested by:	flo, pho
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D23965

sys/x86/x86/io_apic.c

@@ -94,7 +94,7 @@ struct ioapic_intsrc {
 struct ioapic {
 	struct pic io_pic;
 	u_int io_id:8;			/* logical ID */
-	u_int io_apic_id:4;
+	u_int io_apic_id:8;
 	u_int io_intbase:8;		/* System Interrupt base */
 	u_int io_numintr:8;
 	u_int io_haseoi:1;