Correct value for BN_security_bits()

The function BN_security_bits() uses the values from SP800-57 to assign
security bit values for different FF key sizes. However the value for 192
security bits is wrong. SP800-57 has it as 7680 but the code had it as
7690.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4546)

crypto/bn/bn_lib.c

@@ -812,7 +812,7 @@ int BN_security_bits(int L, int N)
     int secbits, bits;
     if (L >= 15360)
         secbits = 256;
-    else if (L >= 7690)
+    else if (L >= 7680)
         secbits = 192;
     else if (L >= 3072)
         secbits = 128;