SDAP: guard RX header parsing with enable_sdap and init per-entity

!3519 (merged) refactored the SDAP to use the QFI from the header rather than pass it to the rx function and got rid of the initialization of QFI to -1.

However, entity roles are stored into the drb2qfi table, and the indexing implies that the QFI is known.

While in TX it is not an issue since packets are forwarded based on the QFI, in RX the QFI can only be parsed from the header, therefore if headers are disabled (--enable_sdap) the QFI is not there and it is wrong to access the header (buf[0]) and looking for it.

This commit adds enable_sdap to nr_sdap_entity_t and initialize it in nr_sdap_add_entity() from sdap->role (disabled when NO_SDAP_HEADER).

In nr_sdap_rx_entity(), only parse buf[0] for QFI and derive sdap_ul_rx/sdap_dl_rx when enable_sdap is true; otherwise, keep offset=0 and forward payload unchanged.