crypto: ecc - Add comment to ecc_digits_from_bytes about input byte array

Add comment to ecc_digits_from_bytes kdoc that the first byte is expected
to hold the most significant bits of the large integer that is converted
into an array of digits.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>