crypto: ecc - expose ecc curves

Move 'ecc_get_curve' to 'include/crypto/ecc_curve.h', so everyone
in kernel tree can easily get ecc curve params;

Signed-off-by: Meng Yu <yumeng18@huawei.com>
Reviewed-by: Zaibo Xu <xuzaibo@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>