env_nand: fix incorrect size parameter to ALLOC_CACHE_ALIGN_BUFFER

The third parameter to ALLOC_CACHE_ALIGN_BUFFER is not size (as named),
but rather count (number of elements of the type to allocate). The
current code ends up allocating one copy of env_t for each byte in its
size, which quite possibly ends up overflowing RAM.

This fixes a bug in commit 3801a15f "env_nand: align NAND buffers".

Reported-by: Prabhakar Lad <prabhakar.csengg@gmail.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reported-by: Prabhakar Lad <prabhakar.lad@ti.com>