keyspan_pda: Fix parse error with as31 version 2.3.1

keyspan_pda.S has an out-of-range decimal constant that looks
like it was really meant to be a binary constant.  Comparing
with the binary, it appears that as31 used to treat this as 0,
so change the source to use 0 and add a comment on this.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

keyspan_pda/keyspan_pda.S

@@ -285,7 +285,12 @@ clear_rx_ring_loop:
 	mov dptr,OUTC
 	movx @dptr, a
 	;; setup the serial port. 9600 8N1.
-	mov a,#01010011		; mode 1, enable rx, clear int
+	;; Original source had:
+	;;mov a,#01010011		; mode 1, enable rx, clear int
+	;; This was presumably meant to be a binary constant, but it's
+	;; really decimal and out of 8-bit range.  as31 used to treat
+	;; it as 0 and that seems to have worked, so carry on with 0.
+	mov a,#0
 	mov SCON, a
 	;;  using timer2, in 16-bit baud-rate-generator mode
 	;;   (xtal 12MHz, internal fosc 24MHz)