crypto: rsa - Remove unneeded error assignment

There is no need to assign an error value to 'ret' prior
to calling mpi_read_raw_from_sgl() because in the case
of error the 'ret' variable will be assigned to the error
code inside the if block.

In the case of non failure, 'ret' will be overwritten
immediately after, so remove the unneeded assignment.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>