ecc: Fix handling of point representation in EdDSA.

* cipher/ecc-eddsa.c (_gcry_ecc_eddsa_ensure_compact): Use
GCRYMPI_FMT_USG, since integer is defined as unsigned in SEC1.
(_gcry_ecc_eddsa_decodepoint): Likewise.

It has been not an issue because it's 2^255 - 19 only (MSB is always
cleared).

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>