Remove fields from BPB that are EBPB actually

This is just a trick so i can use bpb_size in the kernel later,
omitting things that are not relevant to interfacing the FS.

boot/fat.asm

@@ -75,7 +75,7 @@ read:		push ax
 		; dl bit 6-7: cylinder bits 8-9
 		mov cx, dx
 		mov dh, al
-		mov dl, [bp+bpb.drivenum]
+		mov dl, [bp-1]
 		mov ax, 0x0201
 		mov bx, di
 
@@ -128,7 +128,7 @@ resetbuf:	; DS = ES
 		ret
 
 main:		mov bp, params
-		mov [bp+bpb.drivenum], dl
+		mov [bp-1], dl
 
 		; load root directory
 		call resetbuf