Remove unnecessary segment register modification from modify_sector

ponydos.asm

@@ -187,13 +187,8 @@ read_sectors:
 ;  di = 0x0100 for write, 0x0000 for read
 modify_sector:
 	pusha
-	push ds
 
-	xor cx, cx
-	mov ds, cx
-
-	;mov cx, 18
-	mov cl, 18
+	mov cx, 18
 	div cx
 
 	; cl = sector (1…18)
@@ -209,7 +204,7 @@ modify_sector:
 	mov ch, al
 
 	; dl = drive number
-	mov dl, [ds:boot_disk]
+	mov dl, [cs:boot_disk]
 
 	.retry:
 	mov ax, 0x0201 ; read/write one sector
@@ -217,7 +212,6 @@ modify_sector:
 	int 0x13
 	jc .error
 
-	pop ds
 	popa
 	ret