DUART: fix detection of SC26C92 devices

for some reason MR0[3] is always 1 when read; mask this out since all we want to know is if bit 1 was set or not

Source/HBIOS/duart.asm

@@ -693,8 +693,8 @@ DUART_DETECT:
 	LD	A,DUART_CR_MR0		; SET POINTER TO MR0
 	DUART_OUTP(DUART_CR)		; POINTER IS STILL MR2 ON OTHER CHIPS
 	DUART_INP(DUART_MR)		; GET VALUE OF MR0 IN A
-	CP	1			; CHECK FOR TEST VALUE
-	JR	Z,DUART_DETECT_26C92	; YES, MUST BE A '92 WITH MR0
+	AND	1			; MASK TEST VALUE IN BIT 1
+	JR	NZ,DUART_DETECT_26C92	; IF IT'S SET, THIS MUST BE A '92 WITH MR0
 
 	JR	DUART_DETECT_XR88C681	; ASSUME WE HAVE A FANCY EXAR CHIP
 ;