goldfish_nand.c: remove length restrictions

Remove limitations on the size of a read / write request.
AFAIK, there's no consequence to allowing the upper layers
to specify different read / write sizes.

This is needed to support running ext4 on goldfish.

Bug: https://code.google.com/p/android/issues/detail?id=38561
Change-Id: I68a03c67a485e023d4a0fa60100a7b36927ffac0
Signed-off-by: Nick Kralevich <nnk@google.com>