Fix parameter types in sshkdf

Handling of parameter OSSL_KDF_PARAM_SSHKDF_TYPE mixed integer and string
parameters.  This caused endianness problems on big-endian machines.  As a
result, it is not possible to pass FIPS tests since the parameter was stored
with an integer value but read via a cast to char pointer.  While this works
on little endian machines, big endian s390 read the most significant bits
instead of the least significant (as done by, e.g., x86).  Change the
parameter to char array and fix the usages.

Signed-off-by: Juergen Christ <jchrist@linux.ibm.com>

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Patrick Steuer <patrick.steuer@de.ibm.com>
(Merged from https://github.com/openssl/openssl/pull/13781)
