asn1: fix for byte buffer to uint64_t helper

[alexandre-huff]

Hi,

This is a proposal of a fix in the function that converts a byte_buffer to a uint64_t value.

Currently, when the size of a byte_buffer is greater than 1, the code mistakenly converts the bytes into the uint64_t value.

The static_cast<uint64_t>((nbytes - 1 - i) * 8U) erroneously inverts the order of the bits in the resulting value, so that functions void octet_string_helper::to_octet_string(srsran::byte_buffer& buf, uint64_t number) and uint64_t octet_string_helper::to_uint(const byte_buffer& buf) are not equivalent.