lib/mpi: refactor mpi_read_from_buffer() in terms of mpi_read_raw_data()

mpi_read_from_buffer() and mpi_read_raw_data() do basically the same thing
except that the former extracts the number of payload bits from the first
two bytes of the input buffer.

Besides that, the data copying logic is exactly the same.

Replace the open coded buffer to MPI instance conversion by a call to
mpi_read_raw_data().

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>