Volex TTX2000 S Issue 1.2: Routine at 0FA2

0FA2: detect_any_key: Detect if any key is pressed.

Used by the routines at wait_and_decode_page and select_channel.
detect_any_key	0FA2	LD BC,$00FE	Read from all keyboard rows.
	0FA5	IN A,(C)	Read from all keyboard rows.
	0FA7	OR $E0	If no key is pressed then return 0.
	0FA9	CP $FF
	0FAB	JR NZ,detect_any_key_0
	0FAD	XOR A
	0FAE	RET
detect_any_key_0	0FAF	LD A,$FF	Else return $FF.
	0FB1	RET	Else return $FF.