crypto: sm2 - remove unnecessary reset operations

This is an algorithm optimization. The reset operation when
setting the public key is repeated and redundant, so remove it.
At the same time, `sm2_ecc_os2ec()` is optimized to make the
function more simpler and more in line with the Linux code style.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>