fix(arnitech): NET_SEND writes the chip's data port with plain stores

The copy loop wrote each byte with STA RRNET_RX_TX_DATA,X, and an indexed
store reads its address first. Reads of RxTxData take the bytes of a received
frame (AN181), so on a real CS8900A each byte sent took one of a frame the
program had started reading. The loop now stores to the port's low or high
byte by address, as ip65's and Contiki's drivers do; reads stay indexed, which
read once.

src/kernal/arnitech/arnitech_net.s

@@ -273,7 +273,12 @@ arnitech_net_copy:
 	tax
 	bvs @2
 	lda (SAL), y
-	sta RRNET_RX_TX_DATA, x
+	cpx #$01                           ; plain stores: an indexed one reads the port
+	beq @7                             ; first, which takes a byte of a received frame
+	sta RRNET_RX_TX_DATA+0
+	bvc @3                             ; branch always
+@7:
+	sta RRNET_RX_TX_DATA+1
 	bvc @3                             ; branch always
 @2:
 	lda RRNET_RX_TX_DATA, x