Fix a bug in MAPDRV and a bug in the MFRSD kernel.

- MAPDRV was corrupting the register holding the drive number
  while unmapping all the drives in case the device reports
  it has changed. Thus the mapping information ended up written
  to an incorrect memory area.

- The MFRSD driiver was reporting "device changed" even after
  the device had been accessed with DEV_RW. A device is no longer
  considered as "changed" after it has been accessed.

Scenario to reproduce the issue:

- MFRSD with two cards, second one has at least 2 partitions
- On boot, A: is card 1 partition 1, B: is card 2 partition 1
- Right after boot map any drive to card 2 partition 2:
  MAPDDRV d: 2 3 1-3
- Now switch to drive D:, computer crashes

source/kernel/bank4/partit.mac

@@ -3137,7 +3137,7 @@ MAP_SPECIFIC:
 
 MAP_DRV_UNK:
 	;Unknown change status: assume unchanged media.
-	;If it has changed, the boot checksum checksum
+	;If it has changed, the boot checksum
 	;mechanism will do the work of unassigning
 	;all other drives mapped to this device
 	;if necessary.
@@ -3147,6 +3147,7 @@ MAP_DRV_UNK:
 MAP_DRV_CH:
 	push	hl
 	push	iy
+	push	bc
 	ld	e,(hl)
 	inc	hl
 	ld	d,(hl)
@@ -3156,6 +3157,7 @@ MAP_DRV_CH:
 	ld	d,(iy+2)
 	ld	e,(iy+3)
 	call	UNMAPALL
+	pop bc
 	pop	iy
 	pop	hl
 

source/kernel/drivers/MegaFlashRomSD/mfrsd.asm

@@ -666,7 +666,7 @@ DEV_RW:
 
 	;ld	hl,(#f9f0)
 	;inc	hl
-	;ld	(#f9f0),hl	; Cuenta lecturas
+	;ld	(#f9f0),hl	; Count reads
 		
 	call	SD_OFF
 	ei
@@ -677,6 +677,9 @@ DEV_RW:
 	ret
 	
 .ok:	
+	;A successful device access must reset the "device changed" flag
+	res	BIT_SD_CHG,(ix+STATUS)
+
 	xor	a		; Ok
 	ret