destiny: using a bit field for tcp_header->data_offset is more convenient

In the current implementation the data offset is coded into an uint8_t.
Of this uint8_t only 3 bits apply for the data offset.
The remaining bits represent reserved flags for future use.
However, a proper bit masking is forgotten in order
to obtain the data offset part of this uint8_t.

Therefore, defining this uint8_t as a bit field allows a more convenient
method of access.