lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs

Currently, if the number of leading zeros is greater than fits into a
complete limb, mpi_write_sgl() skips them by iterating over them limb-wise.

However, it fails to adjust its internal leading zeros tracking variable,
lzeros, accordingly: it does a

  p -= sizeof(alimb);
  continue;

which should really have been a

  lzeros -= sizeof(alimb);
  continue;

Since lzeros never decreases if its initial value >= sizeof(alimb), nothing
gets copied by mpi_write_sgl() in that case.

Instead of skipping the high order zero limbs within the loop as shown
above, fix the issue by adjusting the copying loop's bounds.

Fixes: 2d4d1eea

 ("lib/mpi: Add mpi sgl helpers")
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>