polio: fix read/writesector

src/mushroom.s

@@ -244,7 +244,7 @@ main:
 4:
     call sendbuffer         # send input buffer
 1:
-    mov ah, 0x07            # do parallel comms
+    mov ax, 0x0700          # do parallel comms
     int 0x80
     mov ax, kRecvBufSize
     push ax

src/polio.s

@@ -70,6 +70,7 @@ readsector:
     mov ch, [bp+2]      # c
     mov dh, [bp+4]      # h
     mov cl, [bp+6]      # s
+    mov dl, 0           # drive
     int 0x13
     ret