CHKIN, CHKOUT: address the channel by the file's secondary address

chkin_iec and ckout_iec read the secondary address from LAT, the table
of logical file numbers, instead of SAT, the table of secondary
addresses. A file whose number differed from its secondary address,
such as OPEN 3,8,2, was read from or written to the wrong channel, and
the read-only and write-only checks tested the file number.

The contract (Michael Steil, "Commodore Peripheral Bus: Part 2",
https://www.pagetable.com/?p=1031): CHKIN sends TALK and CHKOUT sends
LISTEN, each followed by the secondary address OPEN was given; Compute!'s
Mapping the Commodore 64 (p. 52) keeps the logical file numbers at
$0259 (LAT) and the secondary addresses at $026D (SAT). Both routines
now read SAT.

Not reported upstream (https://github.com/MEGA65/open-roms); it may be
what upstream issue #117's OPEN 1,8,0,"$" test met:
https://github.com/MEGA65/open-roms/issues/117

src/kernal/iec/chkin_iec.s

@@ -14,7 +14,7 @@ chkin_iec:
 
 	; Fail if the file is open for writing (secondary address 1)
 	
-	lda LAT, Y
+	lda SAT, Y
 	cmp #$01
 	+beq chkin_file_not_input
 
@@ -24,7 +24,7 @@ chkin_iec:
 	jsr TALK
 	+bcs chkinout_device_not_present
 
-	lda LAT, Y
+	lda SAT, Y
 	ora #$60
 	jsr TKSA
 	+bcs chkinout_device_not_present

src/kernal/iec/ckout_iec.s

@@ -14,7 +14,7 @@ ckout_iec:
 
 	; Fail if the file is open for reading (secondary address 0)
 	
-	lda LAT, Y
+	lda SAT, Y
 	+beq ckout_file_not_output
 
 	; Send LISTEN + SECOND first
@@ -23,7 +23,7 @@ ckout_iec:
 	jsr LISTEN
 	+bcs chkinout_device_not_present
 
-	lda LAT, Y
+	lda SAT, Y
 	ora #$60
 	jsr SECOND
 	+bcs chkinout_device_not_present