fix(arnitech): the network entries also check the PacketPage Pointer

Without a chip the RR-Net's ports read the floating bus, which can read like
the Product ID by chance. The probe now also reads back the PacketPage
Pointer it set to $0000: on a CS8900A its bits C-E always read 011b
(DS271F5 4.10.5), so the high byte reads $30. Seven bytes.

src/kernal/arnitech/arnitech_net.s

@@ -212,8 +212,10 @@ arnitech_net_args:
 	; FALLTROUGH
 
 ;
-; Whether the chip answers: its Product ID at PacketPage $0000. Out: carry clear if it does.
-; X and Y are kept.
+; Whether the chip answers: the PacketPage Pointer, set to $0000, reads back $3000, as its
+; bits C-E always read 011b (DS271F5 4.10.5), and the Product ID is at PacketPage $0000, so a
+; floating bus that reads like the Product ID by chance is no chip. Out: carry clear if it
+; answers. X and Y are kept.
 ;
 
 arnitech_net_probe:
@@ -221,6 +223,9 @@ arnitech_net_probe:
 	lda #$00
 	sta RRNET_PP_POINTER+0
 	sta RRNET_PP_POINTER+1
+	lda RRNET_PP_POINTER+1
+	cmp #$30
+	bne @1
 	lda RRNET_PP_DATA+0
 	cmp #<CS_PRODUCT_ID
 	bne @1