DPP: Use a common helper function for ECDH operations

DPP: Use a common helper function for ECDH operations

This replaces the separately implemented ECDH operations with a single
helper function to avoid code duplication. In addition, this introduces
a workaround for strange OpenSSL behavior where the first
EVP_PKEY_derive(NULL) call to learn the size of the output shared secret
returns unexpectedly large buffer (72 octets when expected 32 octets for
group 19). It is not known what is causing this, but such behavior seems
to be showing up every now and then at least when running hwsim test
cases under UML and apparently mainly (only?) in the sigma_dut
controller cases.

Signed-off-by: Jouni Malinen <j@w1.fi>